The Jonas Brothers took over Kai Cenat 's "Mafiathon 3" stream Friday night ... surprising fans with a performance of "Introducing Me" from "Camp Rock 2" and "Lovebug"!

The famous streamer was just as hyped about his guests as fans were, singing along and at one point gushing over their presence, saying ... "Seeing this in person is crazy, I ain't gonna lie."

Kevin , Joe and Nick clearly had plenty of fun with Kai, but got serious at one point by sharing wholesome, motivational speeches to the chat. Nick told viewers, "You will never regret kindness," while Kevin spoke passionately about the value of family. Play video content

Joe got a bit more playful with his contribution to the tender time in the stream, joking ... "In middle school I s*** my pants, look at me now." B

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