post model of mobo, it may be an am2+ socket board
but if its definately an am2 socket, then no, an am3 chip wont go into it but we could suggest an upgrade dependant on your intended usage of the rig,
Am3 CPUs can work in some AM2 boards. For some it is simply impossible, while for others they need a BIOS update. Even then, I would recommend turning off Cool n Quiet and you may also need to disable VT since AM2 boards were designed to cope with older versions of those technologies and even with a BIOS update may be unstable if you leave those features on.