Only (a) sounds strange. In order for each of them to push Peter out of the car
, Peter would have to fall out of the car and then climb back in so that the next robber could take his turn to push him out again!
("Do it again, I didn't see it!" or a Monty Pythonish "deja vu"
If they all pushed at one time, I say They all pushed...
or just They pushed...
All of the choices in (c) sound OK, but each conjures up a slightly different image in my head.