Subject Verb Agreement

The correct tag question is "aren't I?" in this case because it's a common structure for a sentence with "I am." Thus, the correct answer is "aren't I?"

The error is in part (1). When "together with" is used, the subject is still singular, so the correct verb form should be "is" instead of "are." Thus, the error is in part (1).

The error is in part (3). After "either...or," the verb should agree with the noun closest to it. Since "friends" is plural, the correct verb form should be "have" instead of "has." Thus, the error is in part (3).