I just pregnant for 15weeks. Is it ok to take air plane for 3hours ? I really worry.

Yes you can as long as you are certified safe and healthy. If you need assurance and letter from doctor/gynae, you may let them know and upon booking you may indicate to inform the airline. I did fly for 8 hours flight at 12 weeks and was okay with the flight journey. I didn't do any clearance neither letter request from doctor.