First, you need to stop thinking of each speaker as a ‘system’. It’s just a speaker, and resides on your network.

Each speaker can be set up separately, as a Sonos system, but that isn’t the way they are designed. Each speaker is part of a single Sonos ‘system’ and they can all be grouped so that they are all part of the same “system’ and play the same thing, assuming they’re all part of the same WiFi/subnet. You’d set up the first speaker as your ‘systems, then use the ‘add a device’ on each of the other two, to make them all part of the same system. 

Great!  So, any Sonos speaker can be added to the system and can all be playing the same music, simultaneously!  And I do not need anything other than the speakers (and my iPhone).  That is wonderful news!

And you should have good WiFi coverage at all of the proposed speaker locations.

Just to be clear the speakers below are the only speakers sold by Sonos that are outdoor rated for year-round use. They are wired and must be powered by Sonos Amp also shown. 

The only other speaker that can withstand outdoor use on a limited basis (for your use case) is the Sonos Move 2 which is portable and uses a rechargeable battery with included charger.

Other Sonos speakers while some are moisture and/or humidity resistant are not intended for outdoor use.