Can the pronoun "one" precede the thing it is referring to in a sentence?

I have trouble interpreting the simple declaration "Users are parts of [three] systems." I suspect that the author chose the "Users are parts" language mainly in order to avoid saying "Users use," but the drawback of the non-tautological wording is that it leaves readers to puzzle over what parts of the systems users are.

I think you'd be better off if you made use your primary verb and swapped out "users" in favor of another noun instead—participants, say, or clients or visitors or people or whatever makes sense in connection with the systems involved (which is not made clear in the original sentence).

Once you've made that alteration, the rest of the structure should slide into place much less awkwardly than before. For example:

Participants in the system use not only the service currently under discussion, but also the two services mentioned previously.

As various commenters (most notably John Lawler) have pointed out, putting "one" far ahead of the referent "service[s]" in the sentence does not do the sentence's coherence any favors. If you absolutely reject the idea of saying "service" and "services" in the same sentence, you could rearrange the main phrases so that the pronoun one followed services:

Participants in the system use the two services mentioned previously as well as the one currently under discussion.

But that formulation sounds less natural to me—perhaps because once you turn the sentence's attention to "the two services mentioned previously," they become the ones that are currently under discussion, which makes the subsequent assertion about "the one currently under discussion" seem misplaced.