Can I add a single pole switch to a 3 way switch with power?
Yes, what you drew in the diagram is fine. 

To de-confuse the rats’ nest of wires in the boxes, I would get some colored tape and mark the 3-way travelers (the red/black in the cable between them) with yellow tape, to indicate they are travelers.  Mark both yellow, there is no need to distinguish travelers from each other.  Of course, travelers go on the brass screws on a 3-way.   

You can also mark the black wires in the /2 runs up to the lamp and fan with red tape, to signify that they are switched-hots.