Only soundbars can be used as front speakers in a surround system. Surround systems require two of the same model speaker as surrounds.

You could Group 100’s and 300’s to play the same music.

To use speakers (two of the same modell) as rearspeakers you need a home theater device like Sonos Arc, Beam, Ray or Amp. 
2 Era100 and a single Era300 can be grouped either as two rooms (one as a stereo pair configuration of the two Era100 and the other one as a single speaker setup of the Era300) or as three rooms if all speakers are set up as a single speaker config. 

Your profile says you have a Ray. Use them as surrounds to that.