Installing bathroom exhaust fan (light, fan, and heater) with three seprate switches on wall
Not being there, it is hard to say for sure.  But a fair guess is the the J-box wiring takes care of the heat. This wiring includes the wall stat/timer. (Hopefully this is the 12-gauge wire.)  The other two cables take care of light and exhaust fan. So: j-box cable to red and white in fan.  The other two, take your pick, color to color.