Are both of these sentences correct?
il est parti il y a trois semaines (he left 3 weeks ago)
il est parti pendant trois semaines (he has been gone for 3 weeks)
Your first sentence is right.
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:
I hope this is helpful.
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