Hi,
okay three wires into switch box? hopefully. it will need a 25 amp breaker at the panel, remove the one in the panel and take it with you to the hardware store get a 25 amp replacement. the conduit that runs the old fan should be sufficient or use 16 gauge conduit.
okay locate your power in wire conduit with tester. now all the white neutral wires go together take the black power wires and install it to one side of the switch make a small jumper wire and that must go from the black wire you just installed to the same side of the next switch. the two black wires coming down from the ceiling go to the empty side of the switches, one wire for each switch. depending on what you want, you can use a standard 240 volt wall heater thermostat for the ceiling heater or just use a switch, since those are just mostly used for when your in the bathroom.
if you only have one wire coming down from the fan your going to have to go up in the attic and fish a new wire for the heater and night light nutone makes a special three switch setup for one box for that fan, you might want to buy that, it may come with a wiring diagram too.
any questions write me back, your setup may be different..
ground wires all go together and are green or arent insulated be careful , take time
jm