Not sure I understand your request. You should be ‘bonding’ the Sub with one of those speakers, then grouping the two ‘rooms’ together. 
 

What am I misunderstanding? 

Ignore the sub. The play 1 and era 100 won't stay grouped.

That’s odd. Do you get an error? When do they ungroup? What are the circumstances? Have you called Sonos Support directly to discuss it? 
 

I don’t have any issues grouping ‘rooms’ in my Sonos setup, but my Era 100s are set up as surround speakers, so I don’t normally ‘group’ them. But I frequently group PLAY:1s, PLAY:3s, Roams, and Sonos Ones. Haven’t experienced an issue, so I’m not sure why and where you are having this issue.