make div fill up remaining space

Answer 1

What about something like this? https://jsfiddle.net/Siculus/9vs5nzy2/

CSS:

#container{
    width: 100%;
    float:left;
    overflow:hidden; /* instead of clearfix div */
}
#right{
    float:right;
    width:50px;
    background:yellow;
}
#left{
    float:left;
    width:50px;
    background:red;
}
#remaining{
    overflow: hidden;
    background:#DEDEDE;
}

Body:

    
div3

    
div1

    
div2, remaining