Lenny Dykstra, Conrad Murray, James Debarge -- Jailbird Homies

Conrad Murray and James DeBarge just got a new friend -- Lenny Dykstra is now held up next to them in the L.A. County Jail's celebrity wing.

Dykstra -- who is serving time for assault with a deadly weapon and lewd conduct -- joins the doctor and the singer in the slammer, and we're told at times it has gotten downright raunchy.

We're told Dykstra sometimes puts a sheet over his head -- for fun -- pokes a hole around the waste area and sticks his "baseball bat" through the hole ... for the amusement of anyone who dares to look.

Debarge's lawyer, Spencer Vodnoy, tells TMZ ... Murray and DeBarge are now best buds, and they make fun of Dykstra and his perverted past-time pleasures.

But all good things must come to an end. DeBarge is getting out in a few days.