SELECT u.userId, u.firstName, u.lastName, u.emailAddress, u.phoneNumber, ap.date, ap.time, a.balance
FROM users u WHERE u.firstName = 'first_name' AND u.lastName = 'last_name' AND u.emailAddress = 'email@gmail.com'
INNER JOIN accounts a ON u.userId = a.userId
INNER JOIN appointments ap ON u.userId = ap.userId;
>Solution :
Your query syntax sequence is wrong. If you want to JOIN any table it should come after your parent table.
SELECT u.userId, u.firstName, u.lastName, u.emailAddress, u.phoneNumber, ap.date, ap.time, a.balance
FROM users u
INNER JOIN accounts a ON u.userId = a.userId
INNER JOIN appointments ap ON u.userId = ap.userId
WHERE u.firstName = 'first_name' AND u.lastName = 'last_name' AND u.emailAddress = 'email@gmail.com';
Check on https://www.w3schools.com/mysql/mysql_join.asp