Incorrect Answer.
To see that the sequent
p 
q 
r, q, r 
p.
cannot be valid, we only have to find an assignment of truth values to
p, q, and r, such that p is false whereas the three formulas
p q r, q,
and r are true.
But these constraints determine that p, r get assigned F and q has value T.
These constraints are indeed satisfied, given that
assignment of truth values (the implication is true since its premise is
false; why?). By soundess of the natural deduction calculus
for propositional logic, the sequent above cannot be valid, meaning that it
cannot have a proof.
Back to Question.
Next Question.