If you have a variable (let's call it servoPos) that records the angle of the servo
each button 1 push can increase the value
each button 2 push can decrease the angle
I'm not sure what you want button 3 to do. Where do the fixed angles come from? Wherever they come from the desired angle just needs to be put into servPos.
The servo will just move to whatever angle is saved in the variable using myservo.write(servoPos);
...R