Change default 20px twitter boot click slot for range classes

I use

<div id="windows">
    <div class="row-fluid">
        <div class="span4 mywindow">Text</div>
        <div class="span4 mywindow">Text</div>
        <div class="span4 mywindow">Text</div>
    </div>
</div>

in the div #windows special width.

I want to reduce the default 20px space between these three blocks.

this css code

#windows .span4
{
    margin-right:-10px;
}

sets the correct space between blocks, but each block has the same width, so the total width is less than 100% of the parent div (there is a correct space).

I can fix this by changing the width of the blocks with !important. So the fix

#windows .span4
{
margin-right:-10px;
width:180px !important;
}

The question is: does it make sense to use the Twitter Bootstrap span4 class for my windows if I need to fix this?

, , Twitter Bootstrap, ? Twitter Bootstrap ( span4 - Twitter Bootstrap ).