Changing bathroom light/fan/heater switch
Do all 5 black wires go out of the junction box?
Does one circuit breaker control the fan/heater/light unit? If so:
What you'll need to do, with the electricity on, is check each wire with a multimeter and see which ones have 120V with respect to ground. In other words, hold one probe of the multimeter against the ground wire, then check the other 6 wires. I'll bet that 3 of them will be 'hot'. If the fan-unit was wired correctly, you'll need only one of these hots, and the three wires reading 0 volts will each be going to the fan, heater, and light respectively.
Now, with the power off:
Those three wires reading 0 volts should get wired to the three screws on one side, and one of the wires reading 120 volts should be wired to the single screw on the other side, as well as wiring the ground appropriately.
It seems odd to me though, if it is all one circuit breaker, that the installer would pull down two extra, unneeded wires.