Flights with 2 stops

Cancel as in not show up for it? Assuming it’s on one ticket then NO. The airline will cancel your return flight(s) If you don’t show up for the Munich to Lisbon leg.

You could try to contact the airline to change to a multi city or open jaw ticket. So home to Munich and the Lisbon to home. But that could result in change fees and a fare difference.

Yes, you can change your flight. Contact the airline. It will probably cost $$$ or maybe not. But as noted, don't miss your connection otherwise.

DO NOT skip the final leg. This is taken seriously by the airline.

In 2015, we took LOT (Polish air) to Dubrovnik. We were not logged in to the flight leg from Warsaw to Dubrovnik. 4 weeks later, we did not have a flight home. Fortunately, a very helpful flight desk person found our cancelled flight, and we were able to reinstate it without a penalty. But it took about 2 hours.

If your plan is to get off in Munich, then rethink it quickly. They’d cancel your return from Lisbon. They might work with if you had left the MUN airport and didn’t make it back in time, but they’d have no sympathy if you’re obviously gaming the airfare system.

Multi-city is sometimes referred to as Tri-Fly, as the routes are triangular more or less.

luggage?

Or gate check?

We just do carry-on luggage.

as long as it doesn't get gate checked.... it would put a big dent in your plans

It wouldn't be funny if your luggage was forwarded to Lisbon.

The airlines continue to get more picky about dropping segments of your flight.

I did it in A'dam once. We had a seperate airline ticketed from A'dam to Dusseldorf but ended up leasing a car in A'dam so didn't show for that segment. I was a little worried about my flight home being affected, but when I checked in out of Rome, everything was fine. It wasn't done to save money and the whole family was carry-on only, so baggage wasn't an issue.

Since then, airlines have really cracked down on people dropping segments. I believe it has something to do with potential terrorism (not staying with your bags). I think it also is driven by people booking tickets that are cheaper with a connection than without, then skipping the connection.

If you cancel the leg these days, I'd want to do it formally through a ticketing agent, so they don't cancel the rest of your itinerary. If you have carry-on only, you could probably do it when you get to Munich. If you check bags, however, it could be much more of an issue.