css: correct way to put a div element right down to another div (which is not right next in the html)

css: correct way to put a div element right down to another div (which is
not right next in the html)

Let's say we have something like:
<div id="1" class="A"> </div>
<div id="2" class="B"> </div>
<div id="3" class="C"> </div>
How could we put the div with id 3 right down to the div with id 1?
I tried to do it with negative margins:
.C{
margin-top:-45px;
}
It usually looks good:

But sometimes, if I resize the font with the internet browser, it looks bad: