Some of the units include a heating element, but we didn't need that.I've had fewer maintenance issues with split A/C than with central A/C.

Do you have a significant plumbing project coming up, or do you just want to have a name on hand in case a tenant calls about a toilet backing up or heat not working?

Your best and cheapest bet is making the third floor electric heat only because running and installing gas lines all the way to the third floor will get expensive then you have to get another gas meter with is a lot harder than getting the meter base changed.

my favorite do good for tenant was this:my PM calls  harsh winter tenant just had baby heat is double what it used to be ( gas bill) they cant afford it and rent.. 

I went there many times during the week and addressed all issues including calling a technician for an issue they raised with regards to the heating system.

Will a single Navien Combi-boiler for heat and hot water be enough for all 3 unit?

It provides safe heat and even looks (kinda) like flames.

Agent/MLS, on market for 6 + months in a very heated market where multiple cash bids over asking was normal.

I did explain why my offer was not unreasonable (needs roof, heating systems, etc).  

Heating systems, refrigerators, stoves, water heaters cost the same whether the house is a $30k dive or a $250k middle-class home.