If you have a Touchpad and you have the SDK, you can copy the app from the Touchpad, palm-package it and palm-install it on your Pre 3.
Apparently a mysterious new App Tuckerbox should also allow you to download the Touchpad Facebook app directly to your Pre 3 over-the-air, but it's in beta and the dev is only giving access if you have more than 100 purchased apps. I only have 50, so my only route is via the SDK at the moment.
There are some quirks with it anyway, so the DIY method means you can fix it up before you package and install it on your Pre.
So far I've found the following issues (listing it here since I can't find another thread documenting this):
1) The typical black letterbox bar on the bottom of the screen when in portrait mode
FIX: Edit all 5 instances of index.html - find the <meta name='viewport' content='...> tag, and add this to the start so it reads content='height=device-height, width=...'>
2) In landscape mode, the Photos button doesn't fully show, and you can't scroll
3) Can't upload photos - if you press the Photos button next to the status update text box, eventually a blank popup window appears and the app essentially crashes
FIX: Not sure it's possible to fix - problem is probably because Enyo on webOS 2.x doesn't support the file picker properly