There are several things to consider:
1. How many people are going to use each unit?
2. Will there be laundry equipment in each unit?
3. Will there be inconsiderate tenants using hot water that someone else is paying to heat, who therefore have less incentive to take reasonable showers?
4. What is the cold water supply temperature in summer and winter?
5. Is heating by gas, electric, or oil. Gas and oil have faster recovery so you can use a smaller heater.
6. Are there low-flow (2.5 GPM or less) shower heads in the showers of each unit?
A 50 gallon gas unit will usually serve a modest family if they are reasonable users. Two eighty-gallon units in series for three units would correspond to that.
Heaters larger than 120 gallons usually cost more because they must be made to a Pressure Vessel Code, so if you need that much, you may want to put two smaller heaters in series.
If you give us some more information, you will get a range of suggestions with rationale.