Guild Wars 3 it may be in development at the same team that it was in charge of Guild Wars 2, parts of ArenaNet.
The studio is in fact currently working on a "project not yet announced" and is in the pre-production stages. This project is based on "an established online fantasy IP" and appears to be a new MMORPG game using Unreal Engine 5.
Guild Wars 2: End of Dragons was released in February 2022 and was well received by critics and audiences. Following that success, ArenaNet confirmed that is preparing the title for release on Steam in late 2022 and that a fourth expansion is in the works.
However, a job posting for a new project suggests the development team it would be working on something far greater than a simple expansion. The new title is expected to be developed on Unreal Engine 5, so it seems unlikely that this project is the expansion of Guild Wars 2:
Make your mark on big ideas for an established online fantasy IP by joining a well-funded project in pre-production. […] Take command of the first quick iterations through direct implementation in the Unreal 5 engine and collaboration with team members.
While there is no direct evidence that Guild Wars 3 is actually in development, the fact that the project is tied to "an established online fantasy IP" suggests that ArenaNet both working on a new chapter in the series.
Whatever happens from here until the foreseeable future, the next Guild Wars 2 expansion is likely to arrive well before the launch of Guild Wars 3 - or whatever this mysterious project is - it's probably we will have to wait several more months before having official news about it.