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.

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.