Rusted Relic — ruling 1

If Rusted Relic stops being a creature during combat (because you lose control of one or more artifacts, perhaps), it is removed from combat and will not deal or be dealt combat damage. Creatures it was blocking remain blocked. It won't re-enter that combat, even if you wind up controlling three or more artifacts again during that combat phase.

Rusted Relic — ruling 2

Whether Rusted Relic has "summoning sickness" is based on how long you have continuously controlled it, not on how long it has continuously been a creature.

Rusted Relic : règles, fonctionnement et interactions (MTG)

Rusted Relic is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Metalcraft — This artifact is a 5/5 Golem artifact creature as long as you control three or more artifacts.

How it works : Rusted Relic

Original English text — Wizards of the Coast / Scryfall

#1January 1, 2011· Wizards of the Coast
If Rusted Relic stops being a creature during combat (because you lose control of one or more artifacts, perhaps), it is removed from combat and will not deal or be dealt combat damage. Creatures it was blocking remain blocked. It won't re-enter that combat, even if you wind up controlling three or more artifacts again during that combat phase.
No other card is named in this ruling.
#2January 1, 2011· Wizards of the Coast
Whether Rusted Relic has "summoning sickness" is based on how long you have continuously controlled it, not on how long it has continuously been a creature.
No other card is named in this ruling.