Using multiple buttons on same function that redirects to different functions

Easiest way would probably be to pass in the element into the function:

<button type="button" onclick="myFunction(this)" id="1">Button1</button>
<button type="button" onclick="myFunction(this)" id="2">Button1</button>
<button type="button" onclick="myFunction(this)" id="3">Button1</button>

function myFunction(elem) {
    alert(elem.id);
}

No need to think about event arguments or anything like that.

At its simplest:

<button type="button" onclick="myFunction(this)" id="1">Button1</button>
<button type="button" onclick="myFunction(this)" id="2">Button1</button>
<button type="button" onclick="myFunction(this)" id="3">Button1</button>

function myFunction (button) {
    var x = button.id;
    switch (x) {
        case '1':
            myFunction1(x);
            break;
        case '2':
            myFunction2(x);
            break;
        case '3':
            myFunction3(x);
            break;
        default:
            return false;
    }
}

JS Fiddle demo.

Though I'd amend the above to use unobtrusive JavaScript, moving the JavaScript event-handling from the elements' HTML mark-up:

var buttons = document.getElementsByTagName('button');
for (var i = 0, len = buttons.length; i < len; i++) {
    buttons[i].onclick = function (){
        myFunction (this);
    }
}

JS Fiddle demo.

Or, to make it even easier (and add the event-handling to one element, rather than three):

function myFunction (event) {
    var x = event.target.id;
    console.log(event.target, x);
    switch (x) {
        case '1':
            myFunction1(x);
            break;
        case '2':
            myFunction2(x);
            break;
        case '3':
            myFunction3(x);
            break;
        default:
            return false;
    }
}

var parent = document.getElementById('parentElementID');
parent.addEventListener('click', myFunction);

JS Fiddle demo.

Incidentally, while it's valid (under HTML 5, not under HTML 4) to have an id that starts with a numeric character (0-9), in CSS it's difficult to target those elements (leading numeric characters require escaping, in any one of various ways); so it's still advisable to have a predictable alphabetic prefix to those ids.

References:

• element.addEventListener().
• for () {...} loop.`
• switch () {...} statement.