3 slots is an strange number to have, usually it's either 2 or 4. Since it's an IBM, in my experience, 3 slots means it has a 384mb memory max. Some Aptivas also only work with certain modules. My advice is to go to www.crucial.com
and click on the "Scan My System" button on the right hand of the home page. This tool will tell you what the model number for your computer is and what memory to buy. You can either buy directly or use that info to buy compatible memory from somewhere else, like Newegg.