3 GB Switch is supported for 32 bit editions. It tell operating system to allocate 2 GB RAM to OS and 2 GB RAM to other program such as SQL Or Exchange, if 4 GB RAM is installed.
Well, almost. MSDN (http://msdn.microsoft.com/en-us/library/ms791558.aspx) states
On 32-bit versions of Windows, the /3GB parameter enables 4 GT RAM Tuning, a feature that enlarges the user-mode virtual address space to 3 GB and restricts the kernel-mode components to the remaining 1 GB.
Anyway, thanks for the good question. These are parameters considered rather seldomly which may heavily affect the performance of your system.
Best regards,
Dietmar Weickert.