Comment by Ancapistani

If, like me, you like to script your Mac environment setup, the following two commands SHOULD do the same thing:

    defaults write com.apple.AppleMultitouchTrackpad TrackpadThreeFingerDrag -bool true
    defaults write com.apple.driver.AppleBluetoothMultitouch.trackpad TrackpadThreeFingerDrag -bool true

Unfortunately I've not been able to verify this, as it doesn't take effect until you log out and back in, and I'm in the middle of a task. Once it's done I'll test it and either update this post or comment with my results, depending on how long it takes me.

ETA: I tested it, and the behavior is correct when I log back in after running the above. Oddly though, the settings aren't showing as turned on in the settings panel. If anyone has a scriptable solution to that I'd love to hear it.