Answer:
WHO
1. Who is the tallest in class?
2. Who has the most beautiful mask?
3. Who is Mark?
WHERE
1. Where I was last summer?
2. Where I can find a fast food?
3. Where is Tasmania?
WHICH
1. Which is the biggest: a sofa or a cupboard?
2. Which is the most tough material: wood or stone?
3. Which is taller: the Eiffel Tower or the Big Ben?