3gb switch does not work for 64bit operating systems. The program will use 2gb per proccess unless the application is compiled with the /LARGEADDRESSAWARE switch. In which case it could use 4gb per proccess. SWTOR seems to use 2 proccesses. So i'd guess it could use 2gb each and 4gb total without it being compiled with the /LARGEADDRESSAWARE switch.
So unless i'm wrong please don't use the 3gb switch if you're using a 64bit operating system. However if you're running a 32bit operating system i would recommend trying the 3gb switch.