Question

Three years ago they were at school.Какое это время

Answer 1

Past Simple - поскольку используется ago

Answer 2

Three years ago they were at school. Какое это время?

were  - the Past Simple (Indefinite) Tense

Answer 3

The phrase "Three years ago they were at school" is in the past simple tense. It refers to an action that happened in the past and is now completed. In this case, it indicates that three years prior to the present moment, "they" (a group of people) were attending school.