Weird three divs side by side - css

Question

I'm a tables guy, but I'll need to drag and drop some divs, so I tried doing it tabeless (the right way).
This is what I want to do:
The space between all elements should be 24px. My main problem is having the divs (1,2,3) occupying 100% of available space. The width: 100% its sending them beyond the main container.
This is my code so far:
html
<div id="mainContainer">
<div id="topContainer">Just the top one
</div>
<div id="table">
<div id="Line1Container">
<div id="container1" class="container">1
</div>
<div id="container2" class="container">2
</div>
<div id="container3" class="container">3
</div>
</div>
<div id="Line2Container">
<div id="container4" class="container">4
</div>
<div id="container5" class="container">5
</div>
<div id="container6" class="container">6
</div>
</div>
</div>
</div>
And my css
#mainContainer {
border: 1px solid lightgray;
position:fixed;
top: 80px;
bottom:20px;
left:80px;
right:80px;
overflow-y: scroll;
overflow-x: hidden;
border-top-left-radius: 10px;
border-bottom-left-radius: 10px;
}
#topContainer {
border: 1px solid lightgray;
border-radius: 10px;
margin-left: 24px;
margin-right: 24px;
margin-top: 24px;
}
#table {
display: table;
margin: 24px;
width: 95%;
}
#Line1Container, #Line2Container {
display: table-row;
}
.container {
border: 1px solid lightgray;
display: table-cell;
width: 33%;
border-radius: 10px;
}
As you see I tried the table-cell approach, but before I have tried the float: left approach.
Thanks

Answer 1

Fiddle
You can't properly use px values with % values together with dynamic sizes.
You should use x% instead of 24px.
And you can use float: left on the "cells"

Answer 2

How about using a table for separating the divs? that way with the td padding there will always be 24px between them
check out this fiddle:
http://jsfiddle.net/5zfEq/
added:
#Line1Container {
padding:12px;
}
#inner-table {
width: 100%;
}
#inner-table td {
padding: 12px;
}
based on #Edifice fiddle .... thanks ;)