The point of a super insulated home or a Passive house is that they rely on the body heat of the occupants plus the heat from the various gadgets in the home to keep them warm, with the insulation slowing the heat loss.
A home built to Passive house standard will usually be fitted with hydronic heating (probably as the new owners are not sure that it will do what it says on the tin.) And the heating may never be used.
Of the 37, 000 Passive houses in the world, 20, 000 or so are in Germany where 92% of the owners are satisfied with their economy.
A true Passive house is almost air tight, with a heat recovery ventilation system, fitted with a in duct heating coil to warm the incoming cold winter air, should it be 3.5C or more lower than the room temperature.
You can see from the above that having a large hole in a wall, albeit sealed with a steel sheet, where a vast amount of heat can escape will defeat the whole point of carefully installed insulation.
There are three issues here, which system is better, which system is more energy-efficient and which system is more reliable.
The first issue encompasses all aspects and involves a priority ranking which only you can do.
An issue not brought up by anyone else is the installation. Do you have room for 3 outside units? Does it matter if more space is used or would you prefer to use that space for landscaping or something else?
What about electrical power? Will the installer need to run 3 circuits, one for each compressor? Does your existing breaker panel have the room for 3 circuits?
Another installation consideration: which system is easier to run the linesets (tubes which carry the liquid and gas freon) for?
I expect 3 compressors to cost more than one big compressor. Have you gotten any price quotes?
The one big compressor, assuming it is capable of variable speed, will certainly be more significantly more efficient under anything less than full load because the condenser of a single outdoor unit system has three times the cooling area of a single condenser in a 3 unit system which means the compressor motor doesn't have to work as hard.
The major factor in power consumption is turning the freon back into a liquid and that depends on how fast/easily the condenser can cool it off. All else being equal, a bigger condenser is better (removes the heat faster).
The faster you condense the freon, the less back pressure on the compressor and the less work for the motor.
Most of the time you will not be running full load or all indoor units at once. Even during full load, the big unit will never be less efficient.
The last question is reliability and available. One answer correctly claims that a 3 unit system will be more available but neglects to mention that it will also be more prone to failure because you have more units to fail and each unit is more likely to be run at full capacity while the big unit will usually be running well below its full capacity. Imagine the lifetime of two cars, one run at 90 MPH and one run at 30 MPH.
Best Answer
I can't say anything about the efficiency difference between single unit or minisplit systems for a single room, but for 809 sq ft, 24000 btu (2 tons of refrigeration) seems like a high estimate based on the information here:
http://www.energystar.gov/?c=roomac.pr_properly_sized
http://www.greenbuildingadvisor.com/blogs/dept/musings/calculating-cooling-loads
http://en.wikipedia.org/wiki/Ton_of_refrigeration
Assuming that your concrete block walls are uninsulated and allow air to move through them (true unless there's insulation and an air barrier you didn't mention), the high estimate is probably reasonable. You will save money and energy in the long run, however, if you insulate and seal the walls and ceiling with advice from an energy efficiency/home performance professional, then install an appropriately-sized system.