Are you attempting to run surround sound?

You can “Group” the three AMP’s that will play synchronized audio that is slightly delayed, potentially creating voice sync issues. Only one AMP needs to be connected to the TV in this configuration.

Not really surround sound. Just trying to run right and left sided speakers. But there are 3 speakers on right and 3 on left in a very large room.

How do I run 6 speakers that are not architectural from one Amp? They are 8 ohm speakers that are possibly underpowered if run by a single Amp. 

An impedance matching speaker controller would let you run three on each channel but the power issue might be a problem.

Switching to a Port and external amp would get your speakers working but then you’d have the 75 ms delay issues on the TV sound.

Would it work if a ran an HDMI to an HDMI 1:3 splitter. Each of the 3 HDMI exits would go to 3 different AMPs. Each Amp would run 2 speakers. 
 

Would there be delay and synch issues with this arrangement?

Note it is not an HDMI connection to the Amp - The Sonos Amp needs an HDMI-ARC (audio return channel) connection with CEC protocol - so it’s not a straightforward HDMI link/splitter that you’re looking for in this instance.