You probably don't need any airconditioner. if the temperature outside remains a constant $20-25[^oC]$ then the heat transfer through the walls should cool the room down under $60[^oC]$.
The only reason that might not be if you leave in a country with significant sunshine, and instead of having a good insulation at the roof you have something that heats up.
Instead of an airconditioner, (if you are not concerned about being at 40C, or humidity) I would just suggest that you do a half decent heat instulation at the root and leave it at that.
If you insist on the airconditioner, you need to provide information about the wall heat conductivity coefficients and thicknesses.
Just go give you a very rough indication. Assuming that:
- all walls are exposed to $25^oC$,
- the thickness of the wall is 10 [cm]
- the wall is made out of fire brick (which does not propagate heat that much) with $k = 0.5 [J/mK]$
Then the overall temperature losses from all the side walls and the roof for a uniform temperature of 55[C] will be about 8.4kW.
For the 3[kW], you should expect a (uniform ) temperature difference of about $10 [K]$, i.e. the room temperature will be about $35[^oC]$.
IMHO, you could optionally (even that I think is an overkill):
- add louvres (top and bottom of the side walls) (As kamran suggested)
- add a fan inside the room pointed at the heat source (that will cool better and distribute the temperature in the room)