Edit in JSFiddle

<div class="main">
    <div class="block">uno</div>
    <div class="block">dos</div>
    <div class="block">tre</div>
    <div style="clear: left;"></div>
</div>

.main{
    border: 1px solid red;
}

.block{
    width: 100px;
    height: 100px;
    border: 1px solid black;
    float: left;
}