Luigi's Mansion 3 has been rated for release in South Korea, which suggests that it's coming soon to the Nintendo Switch.
Luigi's Mansion 3 was first announced in the September 2018 Nintendo Direct, where it was revealed that it would take place in a huge building that resembles a hotel. The fans had already guessed that a new Luigi's Mansion game was in development, due to the fact that Luigi was using a new Poltergust weapon during his cameo in the Simon and Richter Belmont reveal trailer for Super Smash Bros. Ultimate. The initial announcement trailer for Luigi's Mansion 3 gave a loose release date of 2019 and no new information has been revealed about the game since.
It seems that the release of Luigi's Mansion 3 might be happening sooner rather than later, as Nintendo has had the game rated by the Game Rating and Administration Committee in South Korea, which is the organization responsible for determining age ratings for games. According to Gematsu, the Luigi's Mansion 3 rating can be found on the GRAC website.
A British retailer called Base.com updated the release dates of several Nintendo Switch games back in February, which included giving Luigi's Mansion 3 a release date of July 19, 2019. It's possible that the July release date was just a placeholder, but the GRAC rating suggests Luigi's Mansion 3 could be coming soon and it's well within Nintendo's character to save a release date announcement until a month before the game hits the stores.
The Nintendo Switch has now outsold the PlayStation 4 in Japan, which shows how much momentum Nintendo has gained over the past two years and how big the potential audience is for a new Luigi's Mansion game. The biggest Nintendo Switch titles of 2019 will almost certainly be Animal Crossing and Pokémon Sword & Shield, but the Luigi's Mansion games have a lot of fans who are craving another console iteration in the series following the release of Luigi's Mansion: Dark Moon for the Nintendo 3DS.
The fact that Luigi's Mansion 3 has been rated for South Korea suggests a release might be imminent. E3 2019 is just around the corner and there are several key games in Nintendo's 2019's schedule that lack a solid release date, including Animal Crossing, and Daemon X Machina. It would make sense for Nintendo to save a Luigi's Mansion 3 release date announcement for their E3 conference, especially if they are planning to release it before the end of the summer.
Luigi's Mansion 3 is due to be released for the Nintendo Switch in 2019.