Three finger tap lookup no longer show Wikipedia pages since upgrading to Sierra
Apple uses Spotlight Suggestions for this functionality since El Capitan: https://support.apple.com/kb/PH21573


  If you’re using OS X v10.10.3 or later and Spotlight Suggestions are enabled in Spotlight preferences, you may also see information about related music, movies, or map locations.


But Spotlight Suggestions are disabled for some reason in most countries: http://www.apple.com/macos/feature-availability/#spotlight-suggestions

Check if your country is in the Supported list. If not, you're out of luck at the moment, just as me. I couldn't find any normal workaround, that doesn't involve constant VPN connection. If you're OK with VPN inconvenience, cost and additional latency, you may check this solution: https://www.reddit.com/r/apple/comments/3cln0j/how_can_you_enable_spotlight_suggestions_in_mac/

Supported countries at the time of writing:


Australia
Austria
Belgium
Canada
France
Germany
Ireland
Italy
Japan
Mexico
Netherlands
New Zealand
Spain
Switzerland
UK
USA