To OP, you can have three available antennas for Rx/Tx but only two are actually used. So the net:net is that when compared to a 3x3:3stream AP, a 3x3:2stream can use all three atnennas to pick the best two, but only use two for Tx/Rx. it will then have 1.5dB less for MRC, TxBF, etc.