My dermatologist prescribed me 4%hydroquinone +tretinoin+mometasone furoate ..for 3 weeks , is it safe to use this combination for such a long duration?

You should always: Believe that your doctor is doing what's best for you, if your doctor says you should combine those for three weeks then you should listen to your doctor. If you are that uncomfortable about it, then i suggest going to another doctor and getting a second opinion. It's hard to get a second opinion here because no one knows your medical/personal history.