Vincent,
Sure. Turn on security history in your server's JDE.INI file -- if you haven't already done so. Then you can either use SQL or write a UBE that cycles through user ID's (F0092) and then look at the security history table (F9312) to find the ones that haven't logged in for the past 3 months. This is a VERY simplified description -- you'll have to do some coding or SQL, but it's definitely do-able.