A solution is to add a hidden element using pseudo-element so you will have 6 elements and keep the layout you need.
.browser {
width: 700px;
display: flex;
justify-content: space-between;
padding-bottom: 100px;
flex-wrap: wrap;
border: 1px solid red;
}
.case {
flex: 0 0 200px;
background-color: #ccc;
height: 100px;
border: 1px solid blue;
margin-bottom: 10px;
}
.browser:after {
content:"";
flex: 0 0 200px;
}
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