Thanks for link to manual.
Solution might take more response for full clarity, and you seem a good electrician.
I have numbered two questions that may be unimportant.
Add a comment, response after 5 pm CST
Here is illustration showing Black Hot and White Neutral in attic junction box, as you described. With black-red-blue wires connected as shown in Broan manual. And each device connected to White neutral wire in junction box.http://waterheatertimer.org/images/Triple-fan-light-night-light.jpg
You have access to attic, and can drop another cable down wall into switch box. This will give necessary total 4 wires between switch and Broanhttp://waterheatertimer.org/OWB1.html
Key start point: Junction box in attic has the Black Hot wire and White neutral coming from circuit breaker box.
This cable powers the junction box.
There are 3 cables in junction box, each cable with black and white.
Cable from breaker box with Hot and Neutral. Cable going to light switch.
Another cable jumps power to next box in the circuit, and will be reconnected when finished, black-to-black white-to-white
Old light switch in wall was single pole.
1) How many cables are inside switch box? Just one cable from attic with black and white wire?
2) 3-way switch is the wrong switch, unless you mean 3-pole triple switch with 3 screw terminals on one side of device?
Here is another link that shows wiring choices if you decide to use 3 separate switches (or timers or occupancy sensor to turn on night light) instead of triple switch: Install triple box.http://waterheatertimer.org/How-to-wire-switches.html#4-f