Old wiring - no hot in switch box - possible switch loop?
I think the problem is that you have the white wire which is the return of the switch leg tied into the neutrals. This creates a direct short when the switch is thrown. 

Find the white that's in the cable for the switch loop, mark it with black tape (because it's technically a hot), and use it as the hot for your fan. Connect the fan's neutral wire to the white bundle. Then you can put all black wires together. 

The third black wire is probably an outgoing branch for another light or outlets. I think this is your scenario (you can reverse black and white in the switch loop if you label the white with tape):



Image attribution