Unless I missed something, it seemed like the only time that they went out on location for Three's Company instead of shooting everything on a soundstage was during the opening credits, when we would see Jack, Janet and company (no pun intended) go to beach, pier, or zoo. We do see an on location shot in the "Moving On" (https://www.threescompany.com/episode/moving-on/) episode, which is actually the pilot for The Ropers (https://www.iamnotastalker.com/2016/09/28/royal-dale-townhouses-from-the-ropers/), but has since been added to Three's Company's syndication package.

https://artworks.thetvdb.com/banners/episodes/77757/257646.jpg

All true but that is fairly common in sitcoms both new and old. Its all shot on a set. Its actually rare to see true outdoor scenes in sitcoms. Besides when the Ropers went looking for a house, we see more outdoor scenes in the Ropers episode when Jenny comes. Mr Roper is in a parking lot talking on a pay phone. Later Mrs Roper and Jenny join Mr Roper in the parking lot and we see Mr Roper drive away.
Mamas Family also had brief outdoor scenes when Mama learns to drive, Mama runs for mayor, and when Mama buys a car.

Single camera sitcoms are more likely to go on location at times. Multi camera ones are mostly always shot on the soundstage.

On the episode "Days Of Beers and Weeds" there was a outdoor scene in the garden. Not sure if that was really outside though. Probably not.

Three's Company had a live audience, which would have made location shots impossible, unless they did away with the live audience for that episode (or for that part of an episode). Compare The Andy Griffith Show (no live audience, lots of outdoor shots) with what was in a lot of ways its "sister" show, The Dick Van Dyke Show (live audience, practically no outdoor shots).

I always thought it would be cool to have an episode or two where they are at the beach shown in the original intro.

In a way it's a relief they never did the cliche Hawaii episode that every sitcom did, I mean just think the trio could barely come up with the monthly rent how would they afford to travel?

On the episode "Days Of Beers and Weeds" there was a outdoor scene in the garden. Not sure if that was really outside though. Probably not.
Yeah that looked shot on a soundstage, the shrubbery was probably decoration but the backdrop looked fake.