Jonas Brothers Join Kai Cenat's 'Mafiathon 3', Sing Throwback Songs

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.

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." But ... he did get serious, too -- encouraging fans to be kind and forgiving.

The JoBros' collab with Kai comes during their "Jonas20: Greetings from Your Hometown" tour, which stops in Los Angeles Saturday evening.

Kai launched "Mafiathon 3" on September 1 -- a month-long livestream that has welcomed some of the biggest names in Hollywood.

Kim Kardashian jumped on for the first day with her son Saint West and made headlines for smashing a table on the back of Kai's head in a hilarious prank.

Ray J crashed the stream after hosting the "TMZ After Dark Tour" ... hopping in Kai's shower in a speedo and sunglasses.

Ice Spice & Latto joined the stream Thursday night and twerked to their new single "Gyatt."