Heads are rolling in USC's athletic department over the college admissions scandal and university fundraising -- a key part of Lori Loughlin and Mossimo Giannulli's defense.

The university just fired 3 officials -- CFO Steve Lopes, senior associate athletic director Ron Orr and associate athletic director Scott Jacobson. USC hasn't said why they were canned, but we've confirmed all 3 were heavily involved in fundraising efforts for Trojan athletics.

TMZ broke the story ... Mossimo wrote a check for $50k, payable directly to USC. Sources connected to the case told us the check was sent to another athletics director, Donna Heinel, who USC already fired. We're told the check was sent at the direction of the scandal ringleader, Rick Singer.

Lori and Mossimo contend they were making a donation on the up and up -- and for that reason, one of their key witnesses could be former Athletic Director Pat Haden.