How to get repeating group to add column/slot when page widens?

My brain isn’t working today and there is probably a really simple solution right in front of me that I’m not seeing, but how do I get this repeating group to expand to allow one more blog post when the page size allows for it?

Nest the rg in a group that is not set to fixed width

Yes, it is set up that way, but I figured out what the problem was - I had the RG set to a fixed number of cells instead of full list. [face palm]

Ugh, so I apparently did not solve my own problem -the RG cannot be set to full list either because on a smaller screen, the RG adds more than 3 posts making it look weird.