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/* If you want to use flexbox for this, you should be able to, by doing this (display: flex on the container, flex: 1 on the items, and text-align: right on .c): */
<div class="main">
<div class="a">First</div>
<div class="b">Second</div>
<div class="c">Third</div>
</div>
.main { display: flex; }
.a, .b, .c {
background: #efefef;
border: 1px solid #999;
flex: 1;
}
.c { text-align: right; }