how can I enable in Preview.app the three fingers gesture that shows the selected word in the dictionary?
This gesture works great in Mail.app but doesn't in Preview.app
Are you viewing a PDF? Unfortunately, the three finger tap gesture does not work with PDF files across the system (e.g. not in Safari as well).
The most convenient alternative I can think of is double tapping a word to select it and then two-finger tapping on that word to show the context menu in which you find the command "Look up word in dictionary".