This has been on the ToDo list for a while - I've not figured out a good way to make the layout editing page easy to use.
In the meantime you can add this CSS to your template's custom CSS file
.categories-list > div {
width: 32%;
float: left;
margin-right: 1%;
}
.categories-list > div:nth-child(3) {
margin-right: 0%;
}
@media (max-width: 768px) {
.categories-list > div {
width: 49%;
float: left;
margin-right: 1%;
}
.categories-list > div:nth-child(2) {
margin-right: 0%;
}
}
This will put your categories in 3 columns with a small space between them. And on narrow devices it will revert to 2 columns