Make child divs expand to fill parent div's width

Answer 1

If you know there will always be three children, you can simply use:

.parent > .child {
    float: left;
    width: 33%;
}

.parent {
    overflow: auto; /*or whatever float wrapping technique you want to use*/
}

If you do not know how many children there are, you will need to use CSS tables, flexbox, or perhaps combine inline-blocks with text-align: justify.