1 Mary was reading a book when the phone rang. Past Continuous. В процесс вклинивается еще одно действие.
2 A few minutes later she was outside. Past Simple ( прошедшее время) , сигнальной слово - a few minutes later. Показано время совершения действия.
3 The sun was shining and the birds were singing as Mary was going to the cinema. Past Continouous, в данном предложении описан процесс.