If you are one of the three, then it's definitely not inappropriate, but it's up to you whether you want to do it or not.
Presumably you view this superior as somewhat reasonable in the sense of 1) likely to listen reasonably open-mindedly and take steps to correct, and 2) unlikely to retaliate.
Obviously, the more secure you feel in keeping your position (or finding a better one), the more comfortable you can feel about this. Still, you'd have to botch things pretty badly or have a very unreasonable superior (in which case you may want to look for a different position) for any kind of retaliation to occur.
If the superior is reasonable, they will probably agree and apologize, and perhaps apologize to the others involved.
Personally, I'm of the mindset that it's unreasonable to be upset when people behave in ways you don't like if you never give them any feedback. Therefore, I tend to lean toward giving people feedback if I'm going to need to continue to interact with them.
That said, I'm very confident in my employability and fairly confident in my social skills in that type of situation.
If you are not one of the three, then it's possible you could bring retaliation upon them if they confided in you as well as be viewed as violating their trust. You should be able to judge this and act appropriately.
(You can ask one of them if they'd mind if you to intervened if you are uncertain.)
It would be better for the feedback to come from them than from you in this case, and I'd encourage them to provide that feedback if they felt comfortable with it.
If you were simply in a place to observe the interaction, then I wouldn't worry about this too much.
In this latter case, though, make sure to make factual claims: just because you observed an interaction that you felt was rude, doesn't mean the parties involved did.
Don't attribute beliefs to people.