Questions about PMA (Pressurized Mating Adapters), three of which are deployed on the ISS.
The ISS has three PMA's installed. PMA-1 is the connector between Unity (node 1) and Zarya, bridging the US and Russian segments.
PMA-2 is attached to the forward CBM port of the Harmnoy (node 2) module. It was used by the Space Shuttle to dock, for most of the stations life. It was shuffled around the station as modules were added to allow continued docking and construction.
PMA-3 is currently attached to the forward CBM port of Tranquility (node 3) and will be moved to the zenith (Space facing) port of Harmony (node 2) to act as a backup docking port for future manned vessels (Dream Chaser, Dragon Rider, CST-100, and Orion).