1) the (I)AP-224 has three diplexed RP-SMA connectors that carry both 2.4Ghz and 5Ghz on the same connectors. However, the ANT-3x3-D608, which is a dual band 3x3 antennas will work, but you will need some kind of RP-SMA-to-N jumper to connect that antenna to that AP. And you are correct, you should connect all three antenna connectors to all three antenna ports on the physical antenna (they are required for MIMO, MRC, etc).
2) The ANT-91 (even if you had three of them) is not recommended as they are not an integrated package and will not give optimal performance. You COULD use a single antenna, which means you would have to provision the AP in 'single-chain' mode (aka only use the ANT0 connector), but you will lose all MIMO, MRC, lower power output and shorter ranged, all the thigns that make the 224 great.
As far as antenna gain, you can use whatever antenna you want, the 1.5dB @2.4GHz and 3dB@5Ghz is stating the internal loss of the AP's diplexor to combine the separate 2.4Ghz and 5Ghz onto the same connector. It's basically stating that compared to the (I)AP-225, there is a bit of additional loss in using external antennas.