in/inside/down

[Allifathima]

1. Putting a live crab in someone's trousers goes beyond a joke.
2. Putting a live crab inside someone's trousers goes beyond a joke.
3. Putting a live crab down someone's trousers goes beyond a joke.
Please correct and comment it.

[Dozy]

These are all feasible, though "inside" seems least likely to me. "down" implies that the crab is introduced from the top (waist), and, normally, that the person is wearing the trousers at the time.