Ryan Garcia says he's "100% sure" he'll knock out Luke Campbell in the first 3 rounds of their January fight ... adding, "I'm very, very, very sure of that."

King Ryan talked all sorts of trash about his opponent on Kevin Hart's "Cold As Balls" YouTube series ... explaining why he believes the fight could propel him to global stardom.

In fact, the 22-year-old told Hart it's downright "the biggest fight of my life so far."

"This is going to put me on, you know, that level where I get that respect," Garcia said. "I think this fight will solidify me as probably one of the best in the world right now."

Ryan is clearly oozing confidence over the upcoming fight ... and perhaps rightfully so, the dude is now 20-0 after knocking out Francisco Fonseca in the 1st round of his last tilt in February.

There's more ... Ryan also talked about his friendship with Canelo Alvarez -- and how they're still on great terms despite Canelo's messy split with promoter Oscar De La Hoya, who still reps Ryan.

"We’ve got the best relationship," Garcia said of Canelo. "We might not have the same language -- he speaks Spanish -- but, man, we get along so good."

Oscar has previously told TMZ Sports he thinks he can bring Ryan a $700 MILLION fight contract one day ... so yeah, no need to burn that bridge, right!?