am3 mobo fits am3 cpu only.
am2+ mobos fit am3,am2+,am2 cpus
am2 motherboards generally work with am3,am2+,am2 as well. Some cpu features will be disabled - like slower HT and also some power saving modes lost.
Also last I checked am3 socket was 941 pins with am3 cpu having 938.
AM3 Sockets are 941 pins and yes an AM3 CPU is 938. Toms did an article about removing two pins from an AM2+ CPU and tried it in an AM3 socket it did not work but it still worked in the Am2+ board with the pins removed. The compatibility between the two goes much deeper then pin layout. Its more to do with the memory controller.