I want to customize my minicart in Magento 2. I need to add 3 attributes

You can create a module to do this. It will use a plugin to add the attributes to the data array which is read by the knockout js template. Then we need to override the template to display these values.

This is the module directory:

|   registration.php
|   
+---etc
|   |   module.xml
|   |   catalog_attributes.xml
|   |   
|   \---frontend
|           di.xml
|           
+---Plugin
|       DefaultItem.php
|       
\---view
    +---frontend
    |   \---layout
    |           checkout_cart_sidebar_item_renderers.xml
    |           
    \---web
        \---template
            \---mini cart
                \---item
                        default.html

catalog_attributes.xml

<?xml version="1.0"?>

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:module:Magento_Catalog:etc/catalog_attributes.xsd">
    <group name="quote_item">
        <attribute name="manufacturer"/>
        <attribute name="part_number"/>
    </group>
</config>

di.xml

<?xml version="1.0"?>

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <type name="Magento\Checkout\CustomerData\DefaultItem">
        <plugin name="AddAttPlug" type="Your\Module\Plugin\DefaultItem" disabled="false" sortOrder="10"/>
    </type>
</config>

DefaultItem.php

<?php

namespace Your\Module\Plugin;

use Magento\Quote\Model\Quote\Item;

class DefaultItem
{
    public function aroundGetItemData($subject, \Closure $proceed, Item $item)
    {
        $data = $proceed($item);
        $product = $item->getProduct();

        $atts = [
            "product_manufacturer" => $product->getAttributeText('manufacturer'),
            "product_part_number" => $product->getAttributeText('product_part_number')
        ];

        return array_merge($data, $atts);
    }
}

SKU already exists in data so no need to add it.

checkout_cart_sidebar_item_renderers.xml

<?xml version="1.0"?>

<page xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:View/Layout/etc/page_configuration.xsd">
    <body>
        <referenceBlock name="minicart">
            <arguments>
                <argument name="jsLayout" xsi:type="array">
                    <item name="components" xsi:type="array">
                        <item name="minicart_content" xsi:type="array">
                            <item name="children" xsi:type="array">
                                <item name="item.renderer" xsi:type="array">
                                    <item name="config" xsi:type="array">
                                        <item name="template" xsi:type="string">Your_Module/minicart/item/default</item>
                                    </item>
                                </item>
                            </item>
                        </item>
                    </item>
                </argument>
            </arguments>
        </referenceBlock>
    </body>
</page>

default.html is a copy of Magento/Checkout/view/frontend/web/template/minicart/item/default.html with changes made at line 66

<li class="item product product-item" data-role="product-item">
    <div class="product">
        <!-- ko if: product_has_url -->
        <a data-bind="attr: {href: product_url, title: product_name}" tabindex="-1" class="product-item-photo">
            <!-- ko foreach: $parent.getRegion('itemImage') -->
                <!-- ko template: {name: getTemplate(), data: item.product_image} --><!-- /ko -->
            <!-- /ko -->
        </a>
        <!-- /ko -->
        <!-- ko ifnot: product_has_url -->
        <span class="product-item-photo">
            <!-- ko foreach: $parent.getRegion('itemImage') -->
                <!-- ko template: {name: getTemplate(), data: item.product_image} --><!-- /ko -->
            <!-- /ko -->
        </span>
        <!-- /ko -->

        <div class="product-item-details">
            <strong class="product-item-name">
                <!-- ko if: product_has_url -->
                <a data-bind="attr: {href: product_url}, text: 
                  product_name"></a>
                <!-- /ko -->
                <!-- ko ifnot: product_has_url -->
                    <!-- ko text: product_name --><!-- /ko -->
                <!-- /ko -->
            </strong>

            <!-- ko if: options.length -->
            <div class="product options" data-mage-init='{"collapsible":{"openedState": "active", "saveState": false}}'>
                <span data-role="title" class="toggle"><!-- ko i18n: 'See Details' --><!-- /ko --></span>

                <div data-role="content" class="content">
                    <strong class="subtitle"><!-- ko i18n: 'Options Details' --><!-- /ko --></strong>
                    <dl class="product options list">
                        <!-- ko foreach: { data: options, as: 'option' } -->
                        <dt class="label"><!-- ko text: option.label --><!-- /ko --></dt>
                        <dd class="values">
                            <!-- ko if: Array.isArray(option.value) -->
                                <span data-bind="html: option.value.join('<br>')"></span>
                            <!-- /ko -->
                            <!-- ko ifnot: Array.isArray(option.value) -->
                                <span data-bind="html: option.value"></span>
                            <!-- /ko -->
                        </dd>
                        <!-- /ko -->
                    </dl>
                </div>
            </div>
            <!-- /ko -->

            <div class="product-item-pricing">
                <!-- ko if: canApplyMsrp -->

                <div class="details-map">
                    <span class="label" data-bind="i18n: 'Price'"></span>
                    <span class="value" data-bind="i18n: 'See price before order confirmation.'"></span>
                </div>
                <!-- /ko -->
                <!-- ko ifnot: canApplyMsrp -->
                <!-- ko foreach: $parent.getRegion('priceSidebar') -->
                    <!-- ko template: {name: getTemplate(), data: item.product_price, as: 'price'} --><!-- /ko -->
                <!-- /ko -->
                <!-- /ko -->

                <div data-bind="html: 'SKU#: ' + item.product_sku"></div>
                <div data-bind="html: 'Manufacturer: ' + item.product_manufacturer"></div>
                <div data-bind="html: 'Part #: ' + item.product_part_number"></div>

                <div class="details-qty qty">
                    <label class="label" data-bind="i18n: 'Qty', attr: {
                           for: 'cart-item-'+item_id+'-qty'}"></label>
                    <input data-bind="attr: {
                           id: 'cart-item-'+item_id+'-qty',
                           'data-cart-item': item_id,
                           'data-item-qty': qty,
                           'data-cart-item-id': product_sku
                           }, value: qty"
                           type="number"
                           size="4"
                           class="item-qty cart-item-qty"
                           maxlength="12"/>
                    <button data-bind="attr: {
                           id: 'update-cart-item-'+item_id,
                           'data-cart-item': item_id,
                           title: $t('Update')
                           }"
                            class="update-cart-item"
                            style="display: none">
                        <span data-bind="i18n: 'Update'"></span>
                    </button>
                </div>
            </div>

            <div class="product actions">
                <!-- ko if: is_visible_in_site_visibility -->
                <div class="primary">
                    <a data-bind="attr: {href: configure_url, title: $t('Edit item')}" class="action edit">
                        <span data-bind="i18n: 'Edit'"></span>
                    </a>
                </div>
                <!-- /ko -->
                <div class="secondary">
                    <a href="#" data-bind="attr: {'data-cart-item': item_id, title: $t('Remove item')}"
                       class="action delete">
                        <span data-bind="i18n: 'Remove'"></span>
                    </a>
                </div>
            </div>
        </div>
    </div>
</li>