The question and answers posed in What is the correct way to pluralize an acronym? deals very nicely with acronym (and possibly initialism) usage. What about the case when you are defining the acronym?
- ...automated teller machine (ATM)... - Easy enough.
- ...automated teller machines (ATM)... ?
- ...automated teller machines (ATMs)... ?
If the singular form will be used later then option 2 seems poor. Generally option 3 appears to be best, but is it clear to the reader that we will write the singular form as "ATM"?
This idea is touched upon in Defining an acronym when using the plural of the term? but it is not complete.