You said the three monisms were equivalent.
Equivalent like 1, 1 and 1 are equivalent?
Or is their relationship more like 1, 0 and -1?
I would say equivalent like 1,1 and 1. Same monism, different terminology. I realised this years ago when I noticed that a textbook definition of Idealism was almost
identical to it's definition of Materialism if you only switched the terms.
Looking at your definition of neutral monism I find it difficult to see how it differs from materialism other than that you use "neutral entity" rather than "matter". I can't see anything inherently non-neutral about the concept of matter.
hammegk's chief objection to materialism is that it implies determinism. Yet no materialist would assume everything is deterministic. So this difference evaporates.
I would contend that any precise, minimally sufficient and internally coherent definition of one monism can be transformed into a sufficient definition of any other monism by simply altering one or two terms.
Even eliminative materialism could be brought into the fold if we simplify entities like "the set of phenomena that is commonly thought of as the mind" into "the mind". All the rest of the phrase is padding.
Of course, this is nothing new, Hume made pretty much the same observations.
There are three of them. Two of them look suspiciously like opposites (they are the two halves of Descartes' dualism)
But since most of us appear to agree that Cartesian Dualism is a dead end we should be prepared to back up and abandon these 17th century pre-scientific categories.
and the other is explicitly neutral.
I think Materialist and Idealist would contend that their basis substance was capable of instantiating every state that exists and is therefore, by definition, neutral.