How to make clamping circuit to clamp exactly at 3.3 volts
The reason that people don't care about the precise clamping voltage is because in most cases, it isn't important enough to take special precautions. Unless you have some really unusual circuit configuration (not revealed) a Schottky diode to ground and another one to the ADC supply voltage should suffice in 99+% of cases.  Most microcontrollers have built-in clamping for signals of moderate impedance.

You may be chasing after problems that don't actually exist.