The /PAE switch is needed to allocate more than 4GB of physical memory (not virtual).
The /3GB switch, instead, changes the default allocation of memory inside the 4GB: from 2GB user space + 2GB kernel space, to 3GB user space + 1GB kernel space
We need the /3GB, together with the patched exe, for FS9. The /PAE and the /3GB switches, are also better not to be used together.