I want to wire four separate sections of lights for my basement from the same breaker. Three sections will be wired to 3 way switches, and one is a single pole. The previous set up was wired from the panel, to the lights and then to a 3 way switch. Maintaining this set up, I have wired Section 1 successfully (14/2 to the lights, 14/3 between lights, 14/2 to switchA and 14/3 to switch B. For the second 3 way switch section, I have 14/3 running ( but not yet connected ) between the 2 switches, and 14/2 from the second switch to the light fixtures and between fixtures. The 3rd 3 way section was already wired and has a 3 way wire coming into the switch box from a switch at the top of the stairs, and a 14/2 going out to the light fixture. I have not yet wired the single pole section. How do I connect the other two 3 way switches and the single pole to the power source?
Your problem is that the run from the 3 switch box to the top of the stairs doesn't have enough conductors. In addition to the hot and two switched you now have, you will need a neutral return for the new single pole section.
You would naturally use two 14/2 here, but there's no sense ripping out the 14/3 that's already there. Just run a 14/2 parallel to it and don't use the black wire.
I'm guessing about which switches must control which lights, but here is a suggested wiring diagram:
B = black, W = white, R = red, where indicated. Wires not marked in the diagram are all switched. Use whatever conductors are available, and please mark the ends of them with a bit of red tape for future maintainers. And don't forget to hook up the equipment grounding conductors!
I have omitted the details of the lights wiring as you have already done these.
Pay special attention to the connections in section 1 as the neutral wire here is carrying the load for the entire basement.
Late Edit: I just noticed that I have recommended what is a code violation in most places. According to modern practice, the run from the 3 switch box to the top of the stairs must either use 14/4 cable, or all the wires must be carried in a single conduit. The reason for this regulation is to avoid inductive heating, magnetic radiation, and excessive line losses.