Q1. Who is Miranda's father?

A: He is Prospero.

Q2. What is the name of the person who opens the gate of Milan to King
Alonso?
A: His name is Antonio.

Q3. He is one of the spirits on the magic island. He helps Prospero and
Miranda. He makes a magic storm and attack the ship. Who is he?
A: He is Ariel.
---------------------
Are the questions and answers all grammatical? Would you check them?

All are correct.

To check the grammaticality of the questions and answers, let's go through each one:

Q1. Who is Miranda's father?
A: He is Prospero.

Both the question and answer are grammatically correct.

Q2. What is the name of the person who opens the gate of Milan to King Alonso?
A: His name is Antonio.

Both the question and answer are grammatically correct.

Q3. He is one of the spirits on the magic island. He helps Prospero and Miranda. He makes a magic storm and attacks the ship. Who is he?
A: He is Ariel.

The question in Q3 seems to be incomplete or missing something. Here's a revised version:
Q3. Who is one of the spirits on the magic island? He helps Prospero and Miranda. He makes a magic storm and attacks the ship.
A: He is Ariel.

With this revision, both the question and answer are grammatically correct.