Ouch, the deeper one thinks about your question, the harder it is to answer. My experience is similar to Greg's -- but I must have overlooked where you said what kind of water heater. Gas or Electric (or some other fuel)? You mentioned you are on one water meter, so that's not a well situation. Presumably you don't have sediment issues otherwise you would be whole-house/building filtering. So that's not a mark against tankless. Yes, for longevity, 15 minutes of maintenance/backflushing of a tankless per year is a good idea. Particularly with well water sources or hard water. Cost is $100 for a recirc pump, 5 gallon bucket, a set of washing machine water hoses and a few gallons of white vinegar. Not expensive.
Personally, I love my NG tankless. I've had it for seven years when I converted from electric 50gal to gas. My next door neighbor has had theirs for 21 years (and hasn't maintained it at all). I have another neighbor who got his the same time we adopted gas -- again, no issues. However -- my examples are all from SFR, not multifamily. I have a house of estrogen -- the missus and two teen
girls. Lots of long showers - before and after my heater conversion.
Now, the showers don't require a rest period in between to keep everyone happy. But I'm on a well -- so no h2o bill. It's less costly now on tankless gas than with the old electric tank. I have tracked my energy use for 20+ years -- a tankless gas unit is very efficient cost-wise, much more so than my electric tank was. And I moved a tank that took up 1/2 of a bedroom closet to an exterior wall where it tanks up very little space.
I used to work in a retail big box store. The electric
water heaters are designed to basically disposable after their warranty
period. Buy a 6 yr unit, maybe get 6-8 years. Buy a 12 yr unit, maybe
14 years, maybe 10. Tankless isn't any different, properly maintained, but they typically have a 20+ year warr. What I saw returned to the retail store was disproportionately electric tanked units (% sold vs % returned). Most of the tanked returns were for bad manufacturing or transport (how do you ship a water heater with smashed controls, dents the size of bowling balls, caved tops, or missing internal piping without a scratch on the shipping carton? Tankless returned was not knowing how to install (some plumbers are thick).
So you have some choosing to do: are you attempting to make your rental units more appealing to current/future residents by having 'more efficient' appliances? Maybe that will translate into a better rental price point? Because the only benefit of going tankless to your tenants is their energy/fuel bill (and endless hot water -- which may counter your cost savings by using hot water longer...)
From my experience, the benefit of going tankless to you beyond marketing would be the probability of longer service use before it rots out, 20+ years -- with the drawback of a bit more maintenance annually or every few years. How long are you going to keep these rental units? If you are flipping this triplex in a handful of years, and you are already plumbed for tanked units, unless you think the new owners would pay more for tankless, or be scared away because tankless is unfamiliar to them -- then factor that in. 'oh, tankless is so much more expensive' -- maybe. How much does a 6yr warranty tank unit cost (x3) vs a 20 year tankless? Be sure to include plumber labor in your calculations.
But you threw in the 'I pay for water' demon. Some people above suggested going with a smaller tanked unit to limit the hot water availability. Unless tenants then get pissed at your unit for not being able to take a 1/2 hr shower. Will that create unwanted turnover?
Damn, you have a puzzle.
Have you considered metering each unit -- put a meter in to each unit feed, maybe a clause in the new rental contract, and going over x gal/mo is a surcharge?