No such thing as a 3GB stick of RAM, to the best of my knowledge.
So you have 2 slots, and 2 sticks of RAM (2GB each)?
One of the sticks must be bad. Boot the laptop with only 1 stick inserted and the second slot empty. If it boots fine, then try booting it with the other stick in the same slot, and again leave the second slot empty. That should help you locate the bad stick. Unless it could be a bad slot, but I don't know how common that would be. There's no reason your laptop shouldn't accept 2 sticks of 2GB each. Depending on your OS, it may not be able to utilize all 4 GB, but it should still work.