Can I write an app in javascript/HTML/CSS and sell it on the app store, google's play,and microsoft mobile app store?

Yes, there is a PhoneGap platform that can be used for building mobile apps for iOS, Android, Symbian, Windows Phone.Apps developed with it are accepted in all the stores. It is quite popular, robust and has nice features.

As mentioned in one of the other answers, yes PhoneGap will do the trick if you decide to write your app in HTML/CSS/JavaScript. Now, if you want to have a pleasant user experience on both smartphone and tablet operating systems, I would also recommend looking into Twitter Bootstrap.

This CSS framework has built-in responsive web-design styles that will allow you to scale things down in a user-friendly manner from tablet to smartphone, as well as show/hide different parts of your HTML depending on whether your users are viewing the page on a tablet or smart phone.

Specifically, you can do things like the following:

<div class="visible-tablet">
    This will only be visible on tablets.
</div>
<div class="visible-phone">
    This will only be visible on smart-phones.
</div>

For a full listing of the responsive design support in Twitter Bootstrap, check out their section on responsive design.

You may also want to check out the Google Chrome Web Developer plugin to assist you in creating a responsive web experience. This plugin lets you (among other things) view how your page will look on multiple device resolutions and is an invaluable tool for debugging resolution-specific issues.

You can look at Zurb Foundation, which is a very active open source project for building responsive web apps for mobile devices. Some overall description of the features here.