Use the “group” feature. You can only pair to create a stereo pair of same-type speakers. 

So is the roam unable to pair/link with say an arc and 2 era 100’s

Since the Roam is a different speaker to the Arc, it cannot be paired to it as I already stated. Ditto with an Era/Roam combination. And, if your Eras are already paired, you can’t “pair” anything else - a pair is quantity 2, not 3. 
 

But

if you want the same music playing from Era, paired Eras and Roam you can use the Group feature to play the same source across multiple speakers/models. You use the word “link”. That’s what grouping does. 

Thanks very much