Responsive Rows / Columns not working

I have six image cards I would like to display on Desktop as 3×2 (columns x rows) and on mobile as 2×3. I created one section, with six columns, and set each Layout Column Width to 50% for Mobile and 33% for Tablet. Works great and seems like the perfect solution for Mobile and Tablet.

But … when I try to set the Layout Column Widths to 33% for Desktop, it won’t let me. (It reduces other column widths so that the total is never more than 100%.)

I tried creating two rows in Desktop, but when I switch to Mobile, understandably I get this:

Box1 Box2
Box3
Box4 Box5
Box6

How am I supposed to create responsive rows?

Cheers, Richard
[Latest version of everything. Using GeneratePress theme]