YMMV
3. Differences?
I will always see polyamoury as it's namesake: multiple-Loves, as in allowing if not encouraging more than one emotionally loving relationships - which may not include sex, kink, or other aspects. Centered more around the relationship between the people, and (hopefully? usually?) with deep mutual involvement. Often these include at least incidental relationships with paramours and sharing information.
In (old) traditions non-monogamy is usually posed as the complement to polyamory, where it focuses more on carnal relations such as sex, swingers, or kink and usually skips emotional involvement or any relating outside of the central activity(ies). But literally non-monogamy is the blanket term for having an intimate relationship with more than 1 person (regardless of type(s) of intimacy) during the same period - so could include/exclude just about every other type of negotiation, including emotional betrayals such as cheating.
ENM is the ethical version, where there is enough negotiation and information-sharing such that no person in a relationship is left out or betrayed in any way: all partners are known (to the degree needed) and accepted by the other partners. That may required full disclosures of who, when, what activities, etc., or just enough to know everyone is safe (ie. STI tests, etc.).
Open Relationships might be seen as a less-involved version, in that there are usually more general rules but perhaps more freedom and less involvement, scheduling, and negotiating with related parties. "Out of sight, out of mind" is more often associated with this, but this type of relationship is NOT strictly defined by that. Your husband could have a boyfriend and a girlfriend, but you don't even know their names nor care as long as hubby is home every night and gets to the important events you schedule together.