Which of the following is correct:

1. She dropped into a sofa.
2. She dropped onto a sofa.
3. She dropped in a sofa.
4. He collapsed on his bed.
5. He collapsed onto his bed.
6. He collapsed in his bed.

Thanks.

ONTO a sofa is correct according to me because sofa is a place to sit. If you are entering into place it is alright to use 'INTO'.
Regards
Lal