You can group up to 32 rooms together. You can pair two speakers in stereo in a single ‘room’ in the Sonos software. 

The Sonos controller in your device (mobile or PC) is merely a remote control, all of the ‘smarts’ are on the computer in each speaker, so if you have three rooms, you can stream three sources simultaneously, one to each room.  

Thank you! So that means I can't link three speakers together in one room?

You can set up three speakers as three separate ‘rooms’ in the Sonos software. Sonos doesn’t care where you put them physically, a ‘room’ is a code construct that Sonos uses. So you can have three speakers, all mono speakers, set up as three rooms, all physically in the same room, or you can have a stereo pair as one ‘room’ and a mono second ‘room’ all playing in sync in the same physical room.