without more specifics, you'll be getting pretty broad or wild answers.
In general, you shouldn't mix brands/speeds of memory together in the same system.
PC133 RAM itself does NOT require it be run with a pair of the exact size. Some motherboards might, but they are few and far between. Just the fact that he has a stick of 512 by itself working lets us know that singles are ok.
Make sure you start filling in your memory slots begenning with slot 1 (or some mobos call it slot 0) and working your way up. Many motherboards, altho they have 3 or more slots, will only support 2 sticks of RAM together. Or, if they support more than 2, they require registered or ECC ram to run stable.
What motherboard do you have? What are the 3 different sticks brand/models? What OS are you running?
My $.02 is that if you're not running windows XP or W2K (which I would guess you're not, due to using 133mhz ram, which hasn't shipped with new computers for quite some time), just use the 512MB stick and dont bother with the extra 2 sticks. Your computer will likely not be able to use it, and often you will just have more headaches/crashes, yet no noticeable performance difference over just the 512MB stick alone.
Good luck.