Discourse 3.5.0.beta8-dev (79ff804509).
I used to use the Add groups to the /about page theme component before it was merged in to core and I was instructed to remove it.
Prior to the merge, my /about page looked like this, with all groups being three columns wide:
Since the merge, staff and moderators are three columns wide but all other additional groups are just two columns wide 
This is the /safe-mode view to confirm it wasn’t a setting I’d changed that might have affected something:
And it continues for all other groups further down the page:
Here are my (core) settings if relevant:
