I'm getting an orthocap treatment to fix gaps between teeth. my lower teeth will move back 3 mm. i'm very worried about how i will look with this overjet. will it be obvious?

Yes: Adding 3mm to your over jet will be usually obvious. One of the disadvantages to clear aligner therapy is less control than braces. You may want to seek a second opinion. Make sure you see a fully qualified Orthodontic Specialist.

Not necessary: if your orthocaps treatment will be carried out by a certified orthocaps practitioner. Treatment with orthocaps yields excellent outcomes in a majority of adult cases.