Rick Blaine wrote:

.....
There's little to no outdoor irrigation allowed, so it's household use only.
Worse case would be less than 10gpm per house I should think. The houses do have
active fire supression (sprinklers), though one wouldn't suspect more than one
house would be using them at a time.
....

With sprinklers (I assume required by building code), the fire codes
and local fire authorities are going to tell you what size pipe
you need, what flow rate, and whether you need to sustain
that rate for just one or all three houses.