There are two fans at the front of the engine and one at the back. Is the back one for the condenser? And when does it kick in?

The rear fan is the 'main' the front two 'auxiliary' fans are also the air-con fans: the auxiliaries will work independently of the air-con and will begin to run at half-speed before the coolant temperature rises high enough for the main fan to also be required.
All three fans (should) have a half-speed and a full speed function for engine cooling.