nope. am2 will fit into an am3 motherboard, but not the other way around.
Actually, AM3 fits in AM2 just fine, but AM2 doesn't go in AM3.
AM2 doesn't work with AM3 motherboards because it lacks the DDR3 controllers, but AM3 contains both the DDR2/DDR3 controllers, allowing it to work in an AM2 motherboard. The only way an AM2 processor can fit in an AM3 motherboard is if that motherboard had DDR2 instead of DDR3.