Within three days you can cover 1-2 location In north India. Hence can pick some good location, create a list and select 1-2 out of them to travel. There are multiple beautiful location in Uttarakhand, Himachal near by Delhi to visit in 2-3 days which you can opt. if heritage & history something fascinate you, Jaipur or Udaipur is right choice to visit in current time. If you inform from where you starting can suggest some specific beautiful location in mountain area which you will enjoy exploring.