Hi I have a 3 hours 15 minutes stay in Instanbul. Can I go out for an hour to meeting a friend also will I need a visa?
You certainly don't have time to leave the airport during your layover. Even if your friend comes to the airport and you get the right visa (or don't need one – you don't mention your nationality so we don't know), realistically you're only going to see them very briefly. To meet your friend, you'll need to go through immigration, and then back through security. Immigration can easily take an hour; security easily half an hour to an hour. You should be at your gate at least half an hour before departure (an hour for a longer flight) and, for safety, you probably want to leave extra time to get through security. That's already filled up most or all of your time with stressful will-I-make-it-on-time queueing. Things get even worse if your incoming plane is late.