Yes, some AM3 motherboards can support older gen CPU's. This is quite normal actually.
I'm not sure exactly what you are confused about. Can you explain further?
M, didn't knew are are two versions of the same model, under two different archictures.The older K8 5200+ are AM2 only, but the K10 version is AM2 and AM3
K8
https://www.cpu-world.com/CPUs/K8/AMD-Athlon 64 X2 5200+ - ADO5200IAA6CS (ADO5200CSBOX).html
K10
https://www.cpu-world.com/CPUs/K10/AMD-Athlon X2 5200+ - AD5200OCK22GM.html
AMD did a lot of this back in that era. A lot of the same cpus with different architecture, tdp, and more. Made it really hard to figure what you needed sometimes.M, didn't knew are are two versions of the same model, under two different archictures.
That explains all.