Is the use of "as", "to be", or "ø" interchangeable in the following context?

In both cases, I find the usage of "as" unsatisfactory and deem it unfit.

Options #1 and #3 are synonymous, but the "to be" in #3 is unnecessary. Use #1.

How does this work?

Well, basically, in this instance, "to find" is a transitive verb (or divalent, if you prefer), and can take a noun phrase argument: "I found the cat," or a complementized (subordinate) phrase: "the terms of the settlement [to be] unsatisfactory." In the complementized phrase, "to be" is non-finite because that's what it does.

A nominal sentence, in English, contains the copular verb "to be," and can have a noun phrase or adjectival argument.

1. You're a [big guy]NP.

2. The flag is [yellow]Adj.

Nominal sentences can drop the "to be" when embedded, like they are here, you can also say "I like my coffee hot," which "hot" obviously isn't an adjective modifying "coffee," or else it would be before "coffee," just like any other English adjective.

So basically, the underlying form of your first sentence is "Shareholders found the terms of the settlement to be unsatisfactory," and the "to be" is dropped because we can do that in English.

The second one is much the same. "Deem" can have a complementized phrase as an argument, see Merriam-Webster's example sentence:

"[D]eemed it wise to go slow"

But here, this isn't a nominal sentence, and you can't drop "to go."

Tl;dr: You don't have to say "to be" in subordinate nominal sentences, so don't use it here.