There are 12 statements and only 4 are true. That gives us 495 combinations. I gave the statements the number 1-12 (1-3 to suspect A, 4-6 to suspect B etc.)

Now i started to eliminate  the combinations that can’t exist:

Statement  1 & 7 can’t  be both true.  Statement  4 & 9 can’t  be both true. Statement  4 & 12 can’t  be both true.
Statement  9 & 12 can’t  be both true. Statement 8 can’t be false with statements 4 or 9 or 12 being true .
Statement 10 can’t be false with statements 9 or 12 being true.
Statement 10 & 8 can’t be both false
Statement 11 can’t be false with statement 4 or 12 being true.
Statement 11 & (5 or 8 or 10) can’t be both false.
Statement 4 &10  can’t be both  true or both false and so 5 & 12  and 9 & 11.
If statement 6 is true than B is not the thief.
If Statements  10 & 11 are true and 12 is false statement 8 can’t be true.

After all the above i was left with 7 possibilities that gave me all the suspects, i notices that in the beginning of the puzzle, it says that the thief did not know that the car was the car of the chief of police, which means that statement 3 must be true.
And that fact  leave us with only one option: false, false, true, false, true, false, false, false, false, true, true, false.

So the thief is Suspect B.