First, let's look at your original sentence:
He seems like a nice guy.
The subject he is singular, and so is a nice guy. Also, a nice guy is indefinite, marked by the indefinite article a.
Now let's look at your examples:
(a) They seem like a nice guy.
Now that the subject's been made plural, it's a little weird that a nice guy is still singular. It sounds like you're considering them all a single person!
(b) They seem like the nice guys.
Okay, good. The nice guys is plural, so it goes with they. The only problem is, you've changed it from indefinite to definite! As you can see in your original sentence, this part of the idiom should be indefinite. The definite article tells the reader that you're comparing them to some specific nice guys. (Who?)
(c) They seem like nice guys.
Now it's the same as the original sentence, except he is replaced with plural they, and the indefinite singular NP a nice guy is replaced with the indefinite plural NP nice guys. This conveys the same thing as the original sentence, except in the plural.
You might have noticed I left out all in (a) through (c). That's because it wasn't relevant, but you can certainly insert it:
(c) They all seem like nice guys.
This sentence is fine.