IIRC the 3GB patch is designed to allow older games to use a full 3 gigs of RAM when they are running, provided you have 3 gigs free and available.
If so, and you only have 3 gigs on your computer total, the game will not have access to all of it even if patched - some RAM has to be available just to have the OS up and running and any other necessary functions. I don't know how much that is, but if it's enough to negate the benefits of patching the game to use more than it currently can, then there would be no point in patching it.
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