PolyOz

Less commonly used. While many people have fairly consistent ideas of the difference between primary (or nesting) relationships and secondary (or non-nesting) ones, the distinction between secondary and tertiary is often more subtle and more idiosyncratic (for those who even use the term). You have to ask what distinction any individual makes, to understand "tertiary".

A somewhat less common usage for primary, secondary and tertiary is for simple ranking. The most involved partner is primary by definition, the less involved is secondary, etc. In this usage, one cannot have two primaries, or a secondary with no primary. Some poly folk tend to avoid these terms entirely because to them it implies a ranking they do not want to endorse, or do not identify with.

A triad is a three way relationship, typically referring to a primary relationship

Three way relationships may be fairly symmetric with all three pairs being fairly equally involved (a triangle); or two of the pairs may be substantially more bonded than the third pair (a vee or V, think of the letter V). These are terms for the ends of a continuum, not binary alternatives. Also, some triads may describe themselves differently in regards to emotional connections and sexual ones, so it's quite possible for example to have a sexual Vee with an emotional triangle.