I have power (12-2 wire) coming to a switch box. That light switch is a 3 way switch (12-3) which goes to the next switch. From there the (12-2) wire goes to the light. I need to connect a light from that light and then down to a single pole switch(12-2 wire). How do I do that?
Short answer you can't.
If what you are describing is correct you only have three conductors in the box attached to the light fixture. That would be a ground, neutral and a switch leg from the 3-way switch. So you do not have a source wire in that box only a switch leg. Your best option to install a separate switched circuit for the second light fixture and run a completely new switch and switch leg.
Hope this helps and good luck.
It just can't be done, not with normal wiring. There are places you'll need 4 wires where you only have 3, and 3 wires where you only have 2.
The only option you'd have would be to use smart switches and control modules up in the light fixtures which are controlled by the smart switches, *and these devices need to communicate via radio or powerline signaling (or both like Insteon). At that point you are simply bussing always-hot (black) and neutral (white) to all locations, having that power the smart switches and control modules, and the smart switches signal the control modules up in the lamp rose to turn the lamp on or off. On the upside, you can turn the lights on/off with your phone. From Kazakhstan.