Got the H3-3D used with the purchase of a crashed P2. It did not have one installed and did not have one in the box. It's definitely necessary, though. It's only $20. However, after fixing up the crashed one, here's how I came out.
Purchased: -$450
Sold Fatshark goggles: +$200
Bought P2 shell new: -$60
Bought new propellers: -$10 (one pair for $7, one pair for $3)
Installed H3-2D on new shell: Free
Sold "refreshed" P2 (with H3-2D & GoPro Hero 3 White): +$600
Sold extra iOSD Mini and Immersion 600mw: +$60
Repair H3-3D that came with crashed P2: -$147
Added GPS booster: -$15
Added Plug & Play cable: -$20
Ordered Hero bus adapter: -$20
New decals: -$20
Died props blue: -$5
Upgraded compass: -$15
So at the end of the day, went from a P2 with H3-2D with one battery, to P2 with H3-3D with two batteries, along with several other upgrades, and also wound up with an extra $113. Filled my gas tank, bought a few nice 22oz bombers of good beers, and took my girlfriend out to dinner for putting up with my "drone stuff" all over the kitchen table for a month. So yeah, $20 for the board still puts me ahead on the build, lol.