Will an AM3 cpu on into an AM2 slot?

It should. Just make sure you have the latest available BIOS on your AM2 board. Otherwise, it might not work at all.

Yes it will fit, however it may or may not work, depending on the board. Some boards it wont other it will with a bios update. To find out go to the boards manufactures website and check the cpu compatibility list for the board.

AM2/AM2+/AM3/AM3+ are all the same physical socket. However, the different revisions of the socket are defined by their motherboard support and vary in power supply, memory connectivity and some other thing I don't know/remember about.

As bignasty said, the best you can do is to navigate to the motherboards manufacturers website and check the cpu compatibility and update the bios in your motherboard as needed.

My own gut feeling is that the answer will be no, your AM3 Sempron 145 won't work in an your AM2 board, if it is only an AM2 and not an AM2+, but the AM3+ board shouldn't be an issue.