Floyd Mayweather "obviously" threw several rounds during the Conor McGregor fight in a "marvelous scam" to set up a rematch ... so says Hall of Fame boxing commentator Jim Lampley.
Lampley -- who's been covering the sport for 3 decades -- UNLEASHES on Mayweather as a scam artist whose essentially praying on "suckers."
But the biggest allegation ... Lampley tells TMZ Sports he's CONVINCED Mayweather intentionally tanked at least 3 rounds in the Conor fight and then carried McGregor to the 10th for show.