Final answer:
The correct imperfect conjugation of the verb 'ser' in the blank would be 'eran' because the subject 'Ellos tres amigos' is third person plural.
Step-by-step explanation:
The correct imperfect conjugation of the verb 'ser' in the blank would be 'eran' because the subject 'Ellos tres amigos' is third person plural. In Spanish, the imperfect tense is used to talk about ongoing or habitual actions in the past. 'eran' is the correct imperfect conjugation of 'ser' for third person plural.
Learn more about Conjugation of Spanish verb 'ser' in the imperfect tense