An unannounced Gundam Battle Operation 3 network test has appeared on Steam
A Steam app called "MOBILE SUITS GUNDAM BATTLE OPERATION 3 Network Test" is live in Valve's database despite Bandai Namco never announcing a third Battle Operation. A Gundam games anniversary stream is due late October.

A Steam entry for a Mobile Suit Gundam: Battle Operation 3 network test has turned up in Valve's database before Bandai Namco has announced the game at all.
What has and hasn't been announced
Battle Operation 3 has never been formally announced. There is no confirmed platform list, release window or developer. Battle Operation 2 has run as a free-to-play mech shooter since 2018 and is still receiving support, which makes a numbered sequel plausible rather than surprising.
Gematsu, which spotted the listing, notes that Bandai Namco has a livestream planned for around the end of October marking the 40th anniversary of Gundam games, and suggests the reveal may have been lined up for it. That is reasonable inference, not confirmation - and a network test registration does not tell you when a test will actually run, or in which regions.
Why it matters
Battle Operation 2 has quietly become one of the longest-running licensed mech shooters, and a PC network test would be its first sign of a sequel. Read the app entry as evidence a build exists, and nothing more.
Why it matters
Battle Operation 2 has run for eight years without a sequel; a registered network test build is the first hard sign one is being prepared, with an anniversary stream lined up to reveal it.