It is possible to use the standard C= A3640 CPU board from an A4000/040 in the CPU slot of an A3000 but there are a number of issues you should be aware of, in particular only certain revisions of the 3640 work well in most A3000s. There is an excelent document that used to be hosted on National Amiga explaining many of the issues,
here is a link to a mirror of it. It also has other useful info such as the correct motherboard jumper positions.
HTH
Robert
