This question type is tricky- it is actually asking us which trees could ever be planted on 1, not which three could be planted at once. That is, maybe W, H, and L could go in 1, just not all at the same time.
When the stem asks us how many or which of the [people/things/trees/movies/etc] are there any which can go in [position] we are not being asked which ones can go together in that position, but which/how many can go there in any circumstance.
