Yes you can combine 3,7,14,21 and etc. day adv fares. Let's say the purchase day is 01JUL. You book a flight for 8JUL LAXLHR using a 7day adv fare and your return is 15JUL LHRLAX.
You have 14day from the return flight for that purchase and of course you are meeting the 7day for the outbound.

This also works on domestic flights.