Princess Island
 

If you have three days in Istanbul, is this worth the time. Which island is the best? Are there any good restaurants?

You mean the Princes' islands? If you only have three days, and you are taking a ferry ride up the Bosphorus, not worth it, given how much there is to see in Istanbul.

My take on my visit:

"Next day I again turned my back on Topkapi Palace and the Grand Bazaar and boarded a packed ferry for the Princes' Islands, so called because inconvenient or disobedient scions of the imperial family were exiled there. While preferable to execution, I found one guide that says that they were blinded first, depriving them of a last sight of Istanbul. And Istanbul seen from the water has to be one of the world's greatest cityscapes.

I had chosen to visit the third of the four islands served by the ferry, Heybeliada, thinking it would be quieter. The big but empty waterfront cafes and rows of horse-drawn carriages waiting for custom suggested the island was usually popular, but not the day I visited. No cars are allowed on the islands, so the locals use the carriages too - or walk or cycle. I started by walking uphill to the Halki Palace Hotel, which Lonely Planet said had a good view from the swimming pool. Not so early in the season, though - the pool was empty, and I ate lunch in lonely state in the dining room.

Hunger satisfied, I strolled more slowly back down to the waterfront, admiring the old wooden houses, in various states of repair, lining the dusty street. After some hard and not particularly successful bargaining I climbed into one of the carriages for a tour of the island. I felt a little guilty as the driver took his two horses up the hills at a gallop, but enjoyed rolling through the pine forests. "

I enjoyed an overnight in the Princes Islands but had a considerably longer stay in Istanbul. With your schedule I'd stay in the city.