It seems to me that "GPS", "FBI", "SMS", and other three-syllable initialisms are examples of molossus, or words with three equally stressed syllables. Are they?
(I've never been good at scansion).
I don't think that nay of these words are molossi (and, yes, that is the plural form of the word). Technically, as acronyms, they are 3 separate words. An easier example would be the term "Hot Cross Buns". You could stress all three words equally, have them all be different, or any combination of stressed and less-stressed words/ syllables. You could say it any way.
In actual usage of the term "GPS", I have noticed that the syllables do tend to be equally stressed. However, in the word "FBI", it depends on the situation. If you are talking casually about the FBI, you may put stress on the 'I' and hold it out a little longer than the other letters. In a news report, though, they may put more stress on the 'F'. This makes a little more sense when you say it out loud, like, "The F BI, today, announced their new plans to do something.", as opposed to: "Oh. I heard that the FBI did something." (with bold letters being stressed)
Note: The first example, with a bold 'F' is representative of the news report, and the second example, with all 3 letters bolded, is representative of a casual conversation.