Seems like you’re using vertical flex-box. Based on the layout you don’t need that. Just create a flex row for the cells with a wrap. The elements will always be the height of the tallest one on the same line.
.imageGrid {
display:flex;
flex-direction: row; /* you don't need this since its the default */
justify-content: space-between;
flex-wrap: wrap;
}
.gridItem {
background-color: #dadada;
flex-basis: calc(33.33% - 24px);
margin-right: 24px;
margin-bottom: 24px;
}<div class="imageGrid">
<div class="gridItem">
<p>stuff</p>
</div>
<div class="gridItem">
<p>stuff</p>
<p>Make this one taller with added stuff</p>
</div>
<div class="gridItem">
<p>stuff</p>
</div>
<div class="gridItem">
<p>stuff</p>
</div>
<div class="gridItem">
<p>stuff</p>
</div>
<div class="gridItem">
<p>stuff</p>
</div>
</div>
