Jameis Winston -- TEAMMATE GROUP SEX ... Runs Rampant In College

Jameis Winston's teammate Chris Casher -- the witness who told cops JW didn't rape his accuser -- also told police he and the FSU quarterback would often share chicks sexually ... and TMZ Sports has learned, that kind of teammate relationship is shockingly common at colleges across the country.

In the police report, obtained by TMZ, cops say Casher stated that when he went into Jameis' bedroom during his sexual encounter with the accuser, he also asked "if [she] would engage in sexual activity with him as well."

Casher explained that kind of situation had happened in the past with other women they had brought to their apartment.

Just a few weeks ago, we also reported on two college basketball players from Oakland University that allegedly double-teamed a woman at a hotel in L.A. while the team was on the road to play UCLA.

We made some calls to a BUNCH of athletes (pro and collegiate) who all told us essentially the same thing -- teammates joining forces to have sex with women at the same time is EXTREMELY COMMON.

One source, who played football at a Big 10 school, tells us, "Running trains happens all the time, sharing girls one after the other."

We also spoke to an athlete who played in the MAC ... who says, "Team threesomes were a good bonding experience. I had a few, but it happened all the time with teammates."

One former college superstar put it another way -- "I used to f**k so many hos and after I was done with her, a b*tch will let everyone else up in her. I mean, of course!"

So, why is group sex so prevalent among college athletes?

As one player put it ... there were ALWAYS groupies offering to hook up with players at the same time. They loved being connected to the football team. And the guys would do it because they felt comfortable around each other and because it was available.

Most of the players we spoke with included this nugget as well -- the teammate group sex thing basically STOPS once you get to the pros.