3. If they (tell) their father, he (be) very angry.

3. If they told their father, he would be very angry.

В условных предложениях второго типа (Type 2) в придаточном предложении (if-clause) используется Past Simple, а в главном предложении (main clause) используется would + инфинитив.

Ответ: If they told their father, he would be very angry.