All the other answers are based on percentage values of the width
property.
I have completely another apporach here. Since you're designing a response
layout you should not rely on percentage values especially when you're trying to fit 3 divs in one row. You should define key resolutions that you're aiming (e.g. smartphone, tablet - landscape/portrait) and design your layout in each of that resolution using media queries.
When using 33%
method you're completely dependent on the device width. You'll never know what the exact width of a div will be so you can't predict how its content will behave.
EDIT:
Approach from your comment might look like this
div.column {
float: left;
width: 33%;
}
@media only screen and (min-width:660px) {
div.column {
width: 220px;
}
}