I have enabled the three-finger lookup, but it is not working since I upgrade to Yosemite. How to fix it?
Are you running Scroll Reverser? It causes the bug. Just quit Scroll Reverser.
The problem is with force touch if you have one of the new macbook pro - if it's on force touch under trackpad -> point and click -> Lookup and data detectors then change it to tap three fingers and it should work.
Have you tried looking into System Preferences > Trackpad? I can't tell if you are referring to Mission Control swipe up with three fingers, but that's definitely still under the menu in trackpad and can be enabled.
I had a similar issue and used these two commands to disable and then re-enable bluetooth. Seemed to revive my issues.
However I have noticed that 3-finger lookup works in some applications and not in others. For instance no problem in my web browsers but not so great in my vm or a few Microsoft products.