If I take 2 x 1500mah 20c 3s packs and wire them in parallel, what do I get?

I know I still get 11.1V, but what happens to my mah and "C" rating?

A 3000mah 20C 3s pack.

Assuming you started with 3s1p packs, now you have a 3s2p.

You do get twice the current or twice the run time as one pack. But the same C rating if you call it a 3000mah pack.

^^^^ Exactly. Lots do the same thing as a way to feed amp hungry setups. Your continuous amps wired in parallel doubled, becoming 20 * 3 = 60amps.

So, if I'm using a 30 amp (40 amp burst) esc and wire two 3s packs in parrallel that are rated for 20c I should be ok?