Html:
<div>
<div class = "firstColumn" >
aaa
</div>
<div class = "subColumn" >
bbb
</div>
<div class = "subColumn" >
ccc
</div>
</div>
CSS
.firstColumn, .subColumn{
border-radius: 8px;
border: 2px solid #18786C;
padding: 5px;
box-sizing: border-box;
float: left;
width: 100%;
}
.firstColumn {
width: 50%;
border-top-right-radius: 0;
border-bottom-right-radius: 0;
}
.subColumn:nth-child(2){
width: 50%;
border-top-left-radius: 0;
border-bottom-left-radius: 0;
border-left: none;
}
.subColumn:nth-child(3){
border-top: none;
}
https://jsfiddle.net/apss/tqap7oc3/3/