Luigi's Mansion 3 was originally the tentative title for the game, prior to becoming the actual title.
The logo looked slightly different, with the "3" being green rather than orange and shaped a bit differently.
The Last Resort looked very different, with a different background.
When Luigi enters the Ballroom, the canister on the Poltergust G-00 is filled with green goo. This does not happen in the actual game, as Luigi does not get Gooigi until after exploring the RIP Suites.
Mario's hotel room, Room 502, looks a bit different.
A poster of Luigi in the RIP Suites hallway, between Room 506 and Room 507, has a much bigger close up than in the real game.
The tablecloths in the Restaurant have a different design, with no food or silverware on them.
There is a clock in place of the key in the Grand Lobby behind the front desk.