Day 3: Take a boat trip to Princess Islands. It will be a rejuvenating tour for you. Visit the biggest island called Buyukada. After traversing the Sea of Marmara by boat, you will arrive in the quaint island and explore this island by horse carriage. It takes 2 hours in the boat from the Istanbul port to the island. The boat sails after every 1 hour, so you can set your plan accordingly. In my opinion, leave early morning so you get the maximum time on the island.
There are limited ferries between Istanbul and Buyukada so make sure you check the ferry schedule before-hand.