pokerfunman
The expense will be duct work, if it does not already exist, with independent cold air returns and hot air ducts for each unit. If they exist, the job is simple and economical. If they do not, the cost will be associated with how difficult it will be to get them run to the proper locations throughout each unit. The short answer is yes, it can be done. The long answer is dependent upon the extend of ductwork realignment. With a crawl space this is easy. Attics can be a pain if not done in fall, or winter. Allow 200 cfm per 6" register, best to have cold air returns in each room.

talon699
You can install ductless units for each unit. Check with your local supplier about them. The evap unit mounts on an inside wall and the cond unit mounts out side. Easier than trying to run ductwork for each apt from the basement to 3 furnace units that you will need/want if you go with ducts. Also a lot of walls will be opened up for ducts. Hope the tenants don`t mind

BigJ
The two downstairs units may not be that difficult, if you have a decent basement to work in. For the 2nd floor though you need to figure in what it's going to take to get up in the ceiling of the 1st floor units to feed the 2nd floor. You're going to want to map out the path with least resistance, meaning the path that you don't have to tear too much apart. You also need to make sure the duct work will fit, or are you going to have to fir out a wall to make it fit and is that worth it? You could probably get a contractor in there for little to nothing if he thinks he's getting the job or has a shot at it to show you the best route and what will need to be done to get everything in.

RICH @www.hvac-for-beginners.com
You have a lot of options from a zoned system to separate units. Probably the best solution will be a ductless system. Go to www.hvac-for-beginners.com, there is all kinds of info there.

Joanne A. W
Years ago, my family bought a house that only had window air conditioners. My dad wanted central air. There was no basement (southern state) so he and the air conditioner company put the rooms' vents in the top part of the walls. It work perfectly and looked nice too.