Agile Manifesto[1], principle #3: Deliver working software frequently, from a couple of weeks to a couple of months, with a preference to the shorter timescale.

There's nothing inherently wrong with this, if 3 months are considered "frequent" in your industry (true for mission/safety critical systems, or systems that should not be upgraded frequently for some reason).

The idea is to deliver small, non-breaking increments to production frequently, rather than infrequent, big bang releases.

1. https://agilemanifesto.org/principles.html