Istanbul

That would be a fast pace for me; you'd really just have to do fly-bys at each site. I just googled layover tours from Istanbul airport and they suggest 6-8 hours to see those places and 1 or 2 smaller ones. If you assume 2 hours roundtrip to/from the airport, that's 4-6 hours.

The Blue Mosque, Hagia Sophia and Topkapi Palace are within 700m of each other. The grounds of Topkapi Palace are quite large, so it takes time just to wander around within it to see the various buildings. It's a twisty, windy 1.2km over to the Grand Bazaar, where you could easily spend hours getting lost in the maze of shops.

So could you "see" them all in 3 hours? Yes. But it wouldn't be my recommendation.

With just 3 hours, I would go for Hagia Sophia, the Blue Mosque and the Basilica Cistern.

Topkapi Palace is close by, but you need several hours there at a minimum. The entry fee is pricey, too. Don't do it unless you can be there long enough to get your money's worth.

The Grand Bazaar is about a 15 minute walk from the Hagia Sophia and Blue Mosque, and then you need some time to explore.

I spent ~4-6 hours just in Topkapi Palace (I had lunch there as well), so I don't recommend trying to see all those sites in such a short time span because it will feel rushed and stressful. I would prioritize so that you can walk away satisfied and, if Istanbul peaks your interest, come back for another visit if you can to see what you missed the first time. I spent over 2 full weeks there and never ran out of things to do, and still have stuff on my list "for next time".

Hello from Wisconsin,
If I were to leave the airport to see one thing...the Aya Sofia. This is one the great buildings of human existence. Entrance is more limited these days so see if you can get in on a tour. See if it is possible to hang around after the tour. Probably not. Turkey has taken a real turn recently under the current president.

The Blue Mosque is quite close to the Aya Sofia. If you are squeezed out of the Aya Sofia, go there next. It is impressive, but it is no Aya Sofia.

Do pay attention as you enter and exit the old city to watch for the Great Wall of Theodosius.

wayne iNWI

Hi all, long time planning to go to Istanbul, searched for a lot of information about the city, as not once was there. I watched various articles and came across this https://passportsymphony.com/travel-guide-top-7-places-to-visit-in-istanbul/ which helped me a lot on my trip to this country. I recommend you read it!