The .sit equivalent for PCs is the .zip file, but the .zip format hasn't changed the way the .sit file format has, so format-incompatibility problems are less common. But if your zip utility is broken or your zip file is broken, you'll have the same problems on a PC that you would with a bad StuffIt Expander or bad .sit file on a Mac.
I'm sorry you keep having trouble. I'm willing to keep the suggestions coming if you haven't given up and thrown the Mac out the window... yet.
Have you installed any unusual software or done any hardware mods to the system since you installed it?
You have a few old Macs, right? Did you try unstuffing on another one of the Macs?
Type 3 errors occur when an illegal instruction is encountered. They can be the result of a corrupt application or a corrupt software component used by that application. Conflicts among Mac OS extensions can be a cause, so you could certainly try reinstalling Mac OS. You could also try booting without extensions (by holding down the Shift key while the system restarts) and seeing if StuffIt behaves better.
Type 11 is officially a hardware exception, but it can also occur from corrupt software or an extension conflict.
Another trick to try: delete all files in the Preferences folder that are named StuffIt anything or Aladdin anything and try unstuffing again.