B450 A pro Max supports Ryzen 5 3600 overclocking?
Pairing a Ryzen 5 3600 with the B450 A Pro Max could allow overclocking, but you'd likely need to add a fan to the A Pro Max's VRM heat sink.
The B450-A Pro seems only slightly sufficient for a high-performance 3900x. The 3600 model draws less power, so I think overclocking should work well.