As far as I can tell, that 3-touch lookup gesture is only available to people with multi-touch trackpads or magic mice.
Furthermore, I personally disable it because it causes unintentional popups frequently. It’s too easy to trigger.
On the other hand, there is another feature that seems to work on an app-by-app basis. And this works with any input device, including mice, graphics tablets, iPad Sidecar, etc. You can ctrl-click on a word, and see a menu that contains “Look Up”. But that doesn’t work in all apps, including Obsidian.
For example, it works in nvAlt, Bear, Scrivener, BusyCal, Things. It works in Mail, Safari or any Apple app. It doesn’t work in Firefox or Obsidian. So it seems to be a very common feature, and it would seem it needs to be enabled somehow by the app.
Safari:

Obsidian:

Firefox doesn’t work:
