<!DOCTYPE html>
<style>
    .multicol {
        width: 500px;
        height: 200px;
        border: 3px solid black;
        -webkit-column-gap: 10px;
        -webkit-column-count: 3;
        line-height: 20px;
    }
    .left, .right {
        padding: 10px 0;
        width: 40px;
        text-align: center;
        box-sizing: border-box;
    }
    .left { float: left; }
    .right { float: right; }
    .tall { height: 220px; }
    .xtall { height: 420px; }
    .blue1 { background-color: skyblue; }
    .blue2 { background-color: dodgerblue; }
    .blue3 { background-color: steelblue; }
</style>

<p>There should be three tall floats, each of which should extend
   into the next column (F2, F3) or into all columns (F1).</p>
<div class="multicol">
    First<br>
    First<br>
    <div class="right tall blue1">F2</div>
    First<br>
    First<br>
    <div class="left xtall blue2">F1</div>
    First<br>
    First<br>
    First<br>
    First<br>
    First<br>
    First<br>

    Second<br>
    Second<br>
    <div class="left tall blue3">F3</div>
    Second<br>
    Second<br>
    Second<br>
    Second<br>
    Second<br>
    Second<br>
    Second<br>

    Third<br>
    Third<br>
    Third<br>
    Third<br>
    Third<br>
    Third<br>
    Third<br>
    Third<br>
    Third<br>
    Third<br>
</div>