There are of course am3 motherboards and cpus.
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.