@Jase - Okay, we aren't talking about the same setting then. But I'm afraid what you describe is not possible. TS3 is a 32-bit only application, and thus by the laws of physics and mathematics, can never use more than 4 GB of RAM. The actual upper limit for TS3 is 3.7 GB due to some overhead. It most certainly can't use 12 GB, if it were forced to do so it would lose its mind and your game would be hosed. And then you would go to jail for breaking the law. (well, really only in a cartoon but anyway...)
Only 64-bit applications can use more than 4 GB.
What is possible to do is reserve that much RAM for the game. Windows tends to not like forced RAM reservations although it might not blink an eye if you have 24 GB to start with. But all that extra memory would then be tied up waiting to be used for something where it can't really be.