Argthtjtr

The first is the correct usage, in my view. The third may be quite acceptable however, since the M in ATM could equally stand for 'machine' or 'machines', though I think pluralising the actual acronym is much clearer in speech.

In any case, never use an apostrophe. 's should only be appended to a word to create the posessive form ("of ..."), never for plurals.

I agree with Wikipedia, wordreference and CMOS - acronyms and initialisms are "regular" nouns; plurals are formed by adding "s".

Checking Google Books for actual usage in a relatively "contentious" case, I searched for:

"OSs" unix windows linux 3120 written instances

"OSes" unix windows linux 1060 instances

"OS's" unix windows linux 520 instances

"Simpler" cases such as CDs vs CD's are even more decisive (over 10:1 in favour of the former).

Oxford Dictionary [e.g. SOS, noun (plural SOSs)] and The Economist [e.g. Are ATMs stealing jobs?] both go for the first option.

Since this is a question about acronyms, and the Federal Government's bureaucracy is notorious for using acronyms, I decided to look up the answer in the United States Government Printing Office (GPO) Style Manual (2000).

Rule 8.11 of the GPO Style Manual states: "While an apostrophe is used to indicate possession and contractions, it is not generally necessary to use an apostrophe to show the plural form of most acronyms, initialisms, or abbreviations, except where clarity and sense demand such inclusion." As examples, the rule suggests:
OKs
ABCs
RIFs
YWCAs

The rule does not show an exception for an acronym, but does refer to one case I found interesting -- the "Oakland A's" needs an apostrophe because otherwise it would be the "Oakland As." From that I would assume that if the addition of an s to an acronym would appear to give the acronym a different meaning, then an apostrophe would be in order. But since acronyms are capitalized letters, the addition of a small s should not make a difference, except where (for some reason) one is writing in a format that is all capitals -- such as the format that military and diplomatic messages were sent until very recently.

I vote for the first, "ATMs". The second is just wrong (apostrophe is not used for plurals, ever). This is because ATM is a defined term for an AT machine, and using it as plural "automatic teller machines" would be a redefinition of a common abbreviation, which one should not be trigger-happy about.

The third, however, does not solve the real need to say there's more than one. It is though clear from the sentence, but might not be so in a general case.

Using 's to pluralize something is called a "Greengrocer's apostrophe".

I think the battle against the Greengrocer's apostrophe is one we're bound to lose - even if grammar of the general population improved, we'd still occasionally be facing nouns which have a mixture of upper and lower case, for which adding an s by itself at the end would be confusing.

Either of the first two is acceptable, but I would recommend the first as the apostrophe isn't needed to convey your meaning, and as such is not required.

The third is just wrong since it creates an awkward sentence that is hard to say and discomforting to read. Most acronyms, including ATM, have a well-defined and commonly accepted meaning, which very rarely includes the pluralization. Avoid the ambiguity and include the s.

Just a small addition to the subject and one that is probably as much related to typography as it is to grammar...

There is an issue with all-caps. For example, if we were:

Talking about ATMs. Then the apostrophe is out.

But, if we were:

TALKING ABOUT ATM'S. Then the case for using one is much stronger as it serves to differentiate Automated Teller Machine from, say, Automated Teller Machine Software

Of course, that doesn't address the question of why you'd be using all-caps in the first place...

Using the apostrophe to indicate plurals of numbers, letters and abbreviations is standard, but it is not as common as s without the apostrophe.