The TK business product is very good, but let down by two things - first IST itself is massively overcrowded, and second the seats are not as flat as some others.
Having said that, 3 hours in IST is more than enough time, and you will love the lounge there which must be one of the best (and largest) in the world. There's so much to do in the lounge that you'll wish you were there longer!
As to the flight, it's ideal for a day flight. The seats are very comfortable for sitting and the food on TK is as good as it gets on an airline.