Your not understanding me, its not the fact that you have a Pentium 3 system, that just tells me you have 242-pin SEC or a 370-pin PGA. Possible a Socket 1 or PGA370, 32KB L1 cache and 256-512KB L@ cache. Processor can run between 450MHz - 1.13 GHz. Doesnt tell me anything about your ram only that your board supports SDRAM.
Are you sure you have an empty memory module?
Your system must support both the technology and form of a memory module.
Now your system will prob use PC 100 but if buy PC 133 it will work but only run at 100.
Heres a link to some different MB of Ram
*edit
I apologize it seems the link doesnt want to work. Go to
www.shopkingston.com, select by memory type, SDRAM PC133, 168-pin Dimm.
PC 133 is cheaper I believe than PC 100, it will still work.