50 Cent has a message for Floyd Mayweather -- YOU SHOULD HATE NELLY'S GUTS ... claiming the rapper banged TWO of Floyd's chicks.
50 just posted a video online -- saying he heard that Floyd talked smack about 3 rappers ... 50, T.I. and Nelly.
Truth is ... Floyd did address certain rappers he's beefing with during an interview with MLive.com -- but didn't mention 50 by name.
He implied that all 3 guys were irrelevant -- saying, "Hip-hop artists, they come and go. They come and go. But I'm still here."
So, 50's response? "Woke up. I look at the computer ... the computer say 'Floyd said f**k T.I., f**k 50, f**k Nelly.'"
"I'm like what he say f**k me for? Nelly f**ked your first baby mama Melissa. Then took your f**ing fiance. [You should] say f**k that n**ga!"
FYI -- Nelly is currently dating Shantel Jackson ... who broke up with Floyd earlier this year.