How can I add a 3-way switch to my light? (confused about existing wiring)
Assuming this is the setup you have described:



You will need to pull a new 12/3 cable to your new 3-way switch. You will also have to replace your existing switch with a 3-way switch. Your wiring setup will look like this:



Because your lighting circuit's power is supplied by a conductor in a 12/3 cable, the figures above were modified as follows:

POWER SOURCE / 2-Wire Romex with Ground (i.e. 12-2) - I modified these to show your 12/3 wire with your RED supplying the power to your lighting circuit (since you described the BLACK in your 12/3 as being spliced into a bunch of other black conductors). From here, everything else should be one-to-one.