Nie podałeś kodu, trudno jest "odgadnąć" z czym dokładnie masz problem.
Przykład z flex.
wersja 1 [ kod on-line ]
<style>
body {
margin: 1rem;
}
.container {
display: flex;
width: 100%;
height: 10dvh;
gap: .25rem;
box-sizing: border-box;
}
.container div {
width: 100%;
height: 100%;
background-color: gray;
color: white;
padding: .5rem;
}
.container .baner {
background-color: green;
}
</style>
<div class="container">
<div class="one">Lorem ipsum 1</div>
<div class="two">Lorem ipsum 2</div>
<div class="baner">Lorem ipsum baner</div>
</div>

wersja 2 [ kod on-line ]
<style>
body {
margin: 1rem;
}
.container {
display: flex;
flex-wrap: wrap;
width: 100%;
height: 10dvh;
gap: .25rem;
box-sizing: border-box;
}
.container div {
flex: 1;
width: 100%;
height: 100%;
background-color: gray;
color: white;
padding: .5rem;
}
.container .baner {
flex-basis: 100%;
background-color: green;
}
</style>
<div class="container">
<div class="one">Lorem ipsum 1</div>
<div class="two">Lorem ipsum 2</div>
<div class="baner">Lorem ipsum baner</div>
</div>

Przykład z grid.
wersja 1 [ kod on-line ]
<style>
body {
margin: 1rem;
}
.container {
display: grid;
grid-template-columns: repeat(3, 1fr);
gap: .25rem;
width: 100%;
height: 10dvh;
}
.container div {
background-color: gray;
color: white;
padding: .5rem;
}
.container .baner {
background-color: green;
}
</style>
<div class="container">
<div class="one">Lorem ipsum 1</div>
<div class="two">Lorem ipsum 2</div>
<div class="baner">Lorem ipsum baner</div>
</div>

wersja 2 [ kod on-line ]
<style>
body {
margin: 1rem;
}
.container {
display: grid;
grid-template-columns: 1fr 1fr;
grid-template-rows: repeat(2, 10dvh);
gap: .25rem;
width: 100%;
}
.container div {
background-color: gray;
color: white;
padding: .5rem;
}
.container .baner {
grid-column: span 2;
background-color: green;
}
</style>
<div class="container">
<div class="one">Lorem ipsum 1</div>
<div class="two">Lorem ipsum 2</div>
<div class="baner">Lorem ipsum baner</div>
</div>
