I am trying to create a flexbox grid which should look like that 
The basic grid layout is pretty easy to create, but I have a very hard time adding the spacing between the elements. As you can see in the code snippet, the spacing in the second line is not working properly.
#flexContainer {
display: flex;
flex-direction: column;
gap: 16px;
height: 200px;
width: 600px;
}
.rowContainer {
display: flex;
flex-direction: row;
gap: 16px;
}
.thirdElement {
flex-grow: 1;
height: 40px;
background-color: red;
}
.twoThirdsElement {
flex-grow: 2;
height: 40px;
background-color: red;
}
<div id="flexContainer">
<div class="rowContainer">
<div class="thirdElement">
</div>
<div class="thirdElement">
</div>
<div class="thirdElement">
</div>
</div>
<div class="rowContainer">
<div class="twoThirdsElement">
</div>
<div class="thirdElement">
</div>
</div>
</div>