Nope! Closed triads and quads (and more) exist. Open relationships are a form of consensual non-monogamy and so are polyamorous ones, but that doesn't mean that open relationship is a synonymous term for polyamory, which is a specific form of ethical non-monogamy.

If you are looking for an umbrella term, it's ethical non-monogamy.

https://www.womenshealthmag.com/relationships/a19664033/polyamory-vs-open-relationships/

"Actually, while the two share some similar characteristics, they’re very different. “An open relationship is one where one or both partners have a desire for sexual relationships outside of each other, and polyamory is about having intimate, loving relationships with multiple people,” says Renee Divine, L.M.F.T., a sex and relationships therapist in Minneapolis, MN."