There used to be a code (way back in the old GTH and skywwalker days) that could force almost a thousand cars into your garage. However, any number over 100 had the same restrictions in buying or winning more, and the extra cars were all 'empty' with the default '0000' part codes or possibly random numbers, I don't remember. In any case, they weren't usable in that form. You could take a GS Pro and edit them directly, but it was a lot of trouble for something that didn't really matter much in the first place and wasn't particularly useful when you could pick up a new memory card for $10, copy your save over, sell a bunch, and buy or trade or win new ones. You could save it to your memory card and the cars you changed would stay the same, but it wasn't good for much beyond hybrid building from the ground up or for bragging rights. I also never trusted it much for the simple fact that all those cars have to be rewriting something, though the game seemed to work just fine.
All of that pretty much REQUIRES an old Gameshark Pro (or another way to edit the garage code, I'm not getting into that) and an old-style PS1 to plug it into; I'd hate to try it with one of the CDX versions that later replaced the GS Pro. But anyway, you're out of luck on the PS3.