Cisco Aironet 1700 Deployment Manual page 41

Figure 54 AP 3500i/e is a 2x3:2 System (Supports up to 2 Transmit Chains)
2x3:2 implies two transmitters and three receivers supporting 2 spatial streams.
Note
Figure 55 Modulation and Coding Scheme: 2SS Bonded Channel Supports up to 300 Mbps
Unlike the AP 3500, the newer AP 3600 supports 3SS with twice as many transmitters (4 per band) enabling faster data rates
of up to 450 Mbps. Note that there is an extra radio for redundancy and enhanced performance (upstream and downstream) and
the AP 3600 can also beamform to 3SS clients as well.
Using a dual-band design, the AP 3600 has a total of 8 transceivers (transmitter/receivers) using only 4 antennas
Four radios are used in each band, 2.4 GHz and 5 GHz.
Cisco Aironet Series 1700/2700/3700 Access Point Deployment Guide
Cisco Aironet Series 3700 Access Point
(Figure 56).