When working with Bootstrap's grid system, it is important to focus on the column layout rather than the row layout. The arrangement of columns will be influenced by the height of the content within each cell.
To illustrate a two-column row example, I have set fixed heights for the rows.
<div class="container-fluid">
<div class="row">
<div class="col col-md-6">
<div style="height: 400px; background-color: rgb(26, 188, 156);"></div>
</div>
<div class="col col-md-6">
<div class="row">
<div class="col col-md-4">
<div style="height: 200px; background-color: rgb(52, 152, 219);"></div>
</div>
<div class="col col-md-4">
<div style="height: 200px; background-color: rgb(231, 76, 60);"></div>
</div>
<div class="col col-md-4">
<div style="height: 200px; background-color: rgb(52, 73, 94);"></div>
</div>
</div>
<div class="row">
<div class="col col-md-4">
<div style="height: 200px; background-color: rgb(22, 160, 133);"></div>
</div>
<div class="col col-md-4">
<div style="height: 200px; background-color: rgb(230, 126, 34);"></div>
</div>
<div class="col col-md-4">
<div style="height: 200px; background-color: rgb(149, 165, 166);"></div>
</div>
</div>
</div>
</div>
</div>