1 furnace 1 water heater 3 apartments

[Guytho Roney]

I have a once triplex converted to a duplex that I’m turning back into a triplex . 
hope that didn’t get too confusing.

So I’m trying to figure out the best way to supply all 3 units with sufficient hot water in the most economical way of course. It’s gas heat and water which I know I may have to pay for unless there is a creative way around that as well.

Looking for some suggestions