Three. One Amp per pair of speakers as each Amp can only control one set of speaker outputs. So three Amps would allow you to group all speakers for ‘party’ mode or play something on just one pair at a time, or two out of three. It *would* allow you to play different music on all three… but if you don’t want that just don’t do it 

One Amp - but - with an impedance matching speaker selector. That would let you select any combination to play together and the impedance matching would bypass the normal Sonos two speaker per channel limit.

You could also use the Sonos/Sonance speakers to get three to play together but you’d still need a switch or volume control to allow you to pick which to play.