Adding a bathroom fan (and speaker) to end line switch
To me it looks like you probably have a  switch leg going from the light to the switch (1 cable with a black and white possibly a ground. That cable the whit should be connected to the hot and the black to the hot for the light when the switch is turned on the black provides the power to the light. If I were making the change I would use “smurf tube” flexible nonmetallic conduit and make the following changes.

All the wires for the circuit have to be in the same cable or to have 3 switches you would need 3 cables , run conduit “smurf tube” and all the wires can be in that conduit.  You said 14 awg wire so I will assume you verified this is a 15 amp circuit. If it is a 20 amp circuit you would need 12 awg. 

I would pull 1 black conductor for the feed to the 3 switches, 1 white conductor for the neutral (code requires the neutral even if not needed for future smart switches).
A ground wire only 1 is needed.
Last I would pull 3 red wires (you can use black just label them so you know 1,2,3). 
Cap the neutral it’s not needed, put the light on 1, fan on 2 and usb on 3 , connecting the devices in the attic with neutral. 

Depending on what code version there may be a way to have a 2nd double switch leg using 14-3 but you need to bring the hot down in this cable also, and return the 2 switched hots and would not have the neutral , there are a few states on older code the only question would they allow this to 1 yoke (3 switches on 1 device it was ok with multi gang boxes and devices and how we did it sometimes prior to needing the neutral. 

The first way is modern code the 2nd may be depending on your jurisdiction.