“I was one of the youngest participants, and only three Americans, invited to participate in the seminar.”
Everyone so far has said that I must write “…and one of only three Americans…”
Everyone so far is correct.
I don’t know about grammatically, but the second form is much clearer.
You do not have to write it that way, but it would be clearer (to me) that way. Also, I’d lose the commas around “and only three Americans.” If you need the commas, you don’t need to be saying what’s between them.
thanks
There’s nothing wrong with either - but I’m tempted to bring it down to this:
I was one of the youngest and one of only three Americans invited to participate in the seminar.
inmo, it is wrong unless you lose that comma, which puts it in apposition, not conjunction. Without the comma, it is not wrong but awkward.
This probably isn’t what you were looking for, but I’m one of those tedious people who insists there can be only one “youngest”, so I’d say “I was one of the younger participants and one of only three Americans…”
Faulty parallelism.
Grammatically, it is fine. But it is difficult to parse and what everyone has been saying you “must” write is probably the preferable version.
There is no faulty parallelism. Two plural noun phrases are being coordinated:
“I was one of [the youngest participants and only three Americans] invited to participate in the seminar.”
You can replace that coordinate structure with either of the two conjuncts and the result is perfectly grammatical:
“I was one of [the youngest participants] invited to participate in the seminar.”
“I was one of [only three Americans] invited to participate in the seminar.”
Why would you insist on something that is false? Wouldn’t you call Bill Gates “one of the ten richest people in the world”? Or would you “correct” that into “one of the ten richer people in the world?”
The above is incorrect because you cannot put a comma before ‘and’ if what follows is an incomplete sentence. Whether you word it your way or ‘their’ way it will still be incorrect. You need to lose the comma.
Better example: I was one of only three Americans invited to the seminar, and I was one of the youngest participants.
You may use a comma before the ‘and’ in the example, but you don’t need it, and I wouldn’t use it. 
Best example: I was one of only three Americans invited to the seminar and was one of the youngest participants.
Brevity and fewer commas make reading easier.
Jesse.
You cannot put a comma before ‘and’ in a sentence if what follows it is an incomplete sentence.
Good sentences convey only one idea. In your sentence you are attempting to convey two ideas. Bad idea. (We’re not picking on fragments at the moment.) - Jess.
There can only be one “youngest”? So you can’t say “the five youngest”?
The fault lies not with the phrases you have underlined being ungrammatical in isolation, but rather lies with those phrases not both being introduced by the preposition “of” to indicate parallelism when used together in a sentence. Since the preposition “of” and the adjective “one” would best not be separated, that leads us to: “I was [one of the youngest participants] and [one of only three Americans] invited to participate in the seminar.”
You can put something after and that is not a complete sentence. For example:
‘I was a small child, and younger than my brother, when I discovered I could beat him to a pulp.’
Nothing wrong with this. (Well, with the sentiment expressed perhaps, especially if you are the brother, but that is neither here nor there.)
To the OP: It might be grammatically ok, but it reads badly.
Gaudere may now beat me with his rule stick.
I think that would depend on the level of the reader. See Spot run, etc.
My d’ruthers? Give me a well coordinated sentence, with effective use of punctuation. Although any given sentence may be longer, the entire composition will be shorter. YMMV depending on reader level.
“I went shopping for slightly nutty Basmati rice, a sweet but piquant Muslim curry, and several russet potatos.”
I can’t believe no one has treated the title as a question about itself, and corrected the error in it, yet.
Ferget Gramma, she’s the cancer anyhow. Sum damn godless feriner’s luring young 'mericans ta some damn preverted semin thang.
To come back to my point about the preposition “of” (along with it’s adjective “one”) being used to introduce parallelism, let’s start with “I was XXX.” “I was [one of the youngest participants].” Without repetition of the preposition (and it’s adjective), that would leave the second element as “I was [only three Americans].” Once “one of” is used to introduce the first parallel element XXX, it needs to be used throughout to introduce each subsequent parallel element.
Nice work! 
I should have been more explicit. Your ‘and’ began the concluding phrase of the sentence, and that’s okay. It’s never all right to put a comma before an ‘and’ as it’s shown in the OP.
I spend my life fighting with grammar nuts/gurus as I write a novel a year. Put ten of 'em in a room with a question and your yield will be ten differing answers. Aaaaaaargh!
Gleena wrote 'Nothing wrong with this. (Well, with the sentiment expressed perhaps, especially if you are the brother, but that is neither here nor there.)
Well, it’s grammatically correct, but… it sucks! (Nothing personal intended, I’m simply illustrating a point.)
Jesse.