He is/was a (former) Commissioner of Police.

I'd say the second one is incorrect.

The first and third are fine and have equivalent meanings.

The second works if he is dead.

True. I was going to say it's still redundant, but I guess "he was a commissioner of police" in that context implies that he had the job when he died. "He was a former commissioner of police" implies that he had been commissioner in the past but no longer held the job when he died.