I don't want to be far fetched in asking this, maybe its too broad, but what's the best subwoofer available for under 3,000
Strictly speaking, that is, picking ONE sub (as opposed to, say, dual PB-13 Ultras), I would nominate the Seaton Submersive. Which is about $2,125 delivered, plus a really good EQ like the SVS EQ-1.
Then too there is a new sub coming from JTR, a revised Captivator that is supposed to perform as well as the original model, but for half the price, and size. Target price is $998, plus an amp and equalization.