Transform into passive: They build houses. / They built houses. / They will build houses.

Решение:

• They build houses. (Present Simple) — Houses are built by them.
• They built houses. (Past Simple) — Houses were built by them.
• They will build houses. (Future Simple) — Houses will be built by them.

Ответ: Houses are built by them. / Houses were built by them. / Houses will be built by them.