/3GB switch not supported in SharePoint - go 64 bit if you need more memory

If you are setting up a 32 bit SharePoint farm, do not turn on the /3GB switch. If you have custom web parts that consume a large amount of memory, or some other need for more memory, you should build a 64 bit farm.

For more info: http://support.microsoft.com/kb/933560/en-us 

If you never have a significant load on your SharePoint environment, you may never experience any problems with the /3GB switch turned on. But under load you may see things like:

  • users get IE page not found error
  • authentication prompts that result in an IE page not found
  • application pools restarting unpredictably