3. They (to send) you a postcard last week, if they (to know) your address.

Решение:

Это условное предложение 2-го типа, которое описывает гипотетическую ситуацию в прошлом. Используем Past Simple в первой части и 'would + инфинитив' во второй.

1. They would have sent you a postcard last week, if they had known your address.

Ответ: They would have sent you a postcard last week, if they had known your address.