All three are correct.
- The wallet which was stolen is mine.
- The stolen wallet is mine.
- The wallet stolen is mine.
Number 1 is a bit long-winded, but is fine. Number 2 is the usual everyday way of saying it. Number 3 is using what's called a postpositive adjective, an adjective that comes after its noun. In English they are usually confined to poetic use, but they are still grammatical. I would expand it to:
The wallet that was stolen is mine.
This is almost identical to sentence number 1.
In The men injured were taken to hospital, you can replace injured with injured in the accident, at which point you can't put it before the noun.
*The injured in the accident men were taken to hospital is wrong
The men injured in the accident were taken to hospital is right
In *The many boys angry refused to stay, angry is not a past participle, whereas both stolen and injured are. We could also say:
The wallet stolen by the monkey is mine.
*The stolen by the monkey wallet is mine.