There was a small thread started about free games inc. AC3 over here:
[Edited by element5, 12/9/2016 8:07:56 AM]
Could be the free version has EasyAntiCheat enabled in the entire game like Watch Dogs 2 since the game's focus is multiplayer. If it's true, then none of the trainers for the retail digital and physical DVD-ROM versions will work.
EDIT: I didn't realize that the trainer for the free version was already released, my bad.
[Edited by samuel_1198, 12/25/2016 9:28:02 PM]