Andre Berto -- Carnival Ridin' With Trey Songz & Chris Brown ... At INSANE Birthday Party

Former 2 -time welterweight champs have all the fun ... just ask Andre Berto -- who spent his weekend turning up with Chris Brown and Trey Songz at Trey's ultra exclusive 30th birthday party.

The carnival themed party took place in Calabasas over the weekend and had a celeb guest list including Terrence J, Draya Michele, and Songz' rumored new chick ... R&B singer Mila J.

Sources close to Berto tell us he thought the theme -- with rides and prizes -- was "one of the dopest party concepts" ever. You can see in this video ... Chris and Trey rode bumper cars, and also did an impromptu concert.

The party was a tough ticket ... we're told there were only 75 guests. Hollywood-speak for intimate.