Leviton 5245 3 way combination switch and receptacle

SOURCE: I have a leviton combination (2) switch... I want

Don't break away the tab.
It sounds like you have Hot wire connected to wrong side of device.

Electricians don't guess, they test.
Separate wires. Turn power ON.
Test each wire to bare ground.
Tester lights up on Hot wire.

Hot connects to brass screw(s). This will feed power into each switch.
Wires going to Load (lights) connect to each silver screw.

SOURCE: I have a 12/2 (black/white/ground)

The switch that you have works like 3 seperate switches. most of these switches will have gold screws
on one side and silver screws on the other side, lets say that the left is gold and the right is silver, and
the switch may also have a green screw for ground. your 12/2 with ground is your main hot, you may
want to mark it with a piece of tape, so you know it's the incoming hot. the ground off your hot will tie,
to the green screw on switch and continue and tie to your vent light which should also have a ground
the white coming off the hot is your neutral, it will go straight to your white or whites on your heater.
the black hot wire at your switch will go to each gold terminal on one side of the switch,there are two
ways you can do this, go to one side and just run a jumper, or put one wire on each gold screw and
tie in to your hot-black wire with a wire nut. now for the switch legs, you will need to run from your, switch
to your heater either another pair of 12/2s or a 12/3, each side of the switch that has one silver screw,
it's easier with a 12/3 normally a 12/3 will have a red,black,white,and ground.
top switch red to red on heater,black to black on light and the white blue on the vent,so from your switch
to the heater you would have to run a12/2 you would not use the black only the white and ground.
remember the white-neutral coming from the switch will be tied to all the whites at the vent,light,heater
except for the 10/3. the 10/3 is used stricktly for the switch legs. good luck!