"How can i make that my robot will always be in the center when it is in straight line."
I don't quite understand this, and how it connects to the rest of your question, but...
If your robot is overshooting the line, simply tell it to turn more sharply. Adjust the code if you are using an MCU. I used a straight line arrangement and it worked fine.
If your sensors show this-
011- you will know you are hitting a 90 degree angle.
If your sensors show this-
001- you will know it is not as sharp a line.
Three sensors are limiting though. Try five.