The third radio (radio2/Marvell 88W8887) is a 1x1:1 internal antenna. In the stock Linksys firmware, it is used for detection of radar emissions from Weather Doppler Radar, Aeroports, Military Installations, & Amateur Radios as part of its DFS (Dynamic Frequency Selection) certification for operating at 5GHz CH 52-144. People don't really recommend using it for anything other than IoT or low bandwidth devices given that it has a relatively short range.
Disclaimer: I do not own the 3200ACM. I own the 1900ACS and build custom OpenWrt images for it, reading the forums for researching what settings and features/optimisations to put in my custom image often lead me to threads with other WRT AC devices.