Bonjour Lorraine,

Your first sentence is right.

However: 

Il est parti pendant trois semaines = he left for three weeks (he is back now)

Il est parti depuis trois semaines = he has been gone for three weeks (he is                                                                                                          still away)

Here are some links to explain the rules:

expressing-for-duration-with-either-pendant-durant-depuis-or-pour

how-you-use-depuis-since-for-with-le-present-and-not-le-passe-compose-prepositions-of-time

I hope this is helpful.

Bonne journée!