3. Mis hermanos y yo hablamos con los abuelos todos los domingos. (Sus/ Nuestros) abuelos son muy buenos.

Answer

The correct answer is 'Nuestros'. In Spanish, possessive adjectives must agree with the possessor and the noun they modify. Since the first sentence says 'Mis hermanos y yo' (My brothers and I), the possessors are 'nosotros' (we). The possessive adjective for 'nosotros' is 'nuestro/a/os/as'. Because 'abuelos' (grandparents) is masculine and plural, the adjective must also be masculine and plural, resulting in 'Nuestros'. The alternative 'Sus' would mean 'Their' or 'Your (formal)', which does not match the subject 'we' established in the first sentence.