I sat for last tashahhud for Isha then when I said 'at' I felt 100% sure that it is the third rakah so I stood up and continued praying. I didn't want to start praying again because I was afraid that I will miss Isha and also because I have waswasah about my salah not being valid. After I finished felt that I done an act of kufr because the time for isha might have been up. After about 15mins, I did sujud assahw and then I repeated that salah. Was what I done kufr according to any opinion? I didn't know what to do if someone makes this mistake and I remember reading that it is haram to terminate fardh salah.

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

What you did has nothing to do with disbelief at all. In fact, you standing upon realizing that it was only the third Rak’ah was the correct thing to do. The only thing you had to do was to prostrate for forgetfulness at the end of your prayer. Your prayer was correct and you did not have to redo it.

Therefore, shun these whispers and do not pay attention to them, because giving in to whispers leads a great evil.

Allah knows best.