I want to control a PSU with an Arduino. but for extra safety, I want to make it an over/under voltage protection circuit with Arduino. One of the 3.3V , 5V, and 12V wires will be spliced and connected to the measuring circuit. If Arduino sees an over/under voltage condition it should pull the pin connected to the PS_ON relay low (thus pulls PS_ON high and turning off the power) and read "[condition]\nDetected on [rail]". (Ex. "Overvoltage\nDetected on 5V") on the LCD unless "Disable OVP" and/or "Disable UVP" switch is enabled in the Arduino Manager on the iPhone.
If they are all +ve voltages then use voltage dividers to bring each into the 5V range (allowing for any overvoltage conditions of course). Then monitor these analog inputs for problem voltages...
What is your question?
How can the above be done?
You can connect 3.3V directly to Arduino's analog input like A0.
Or connect 5V through a voltage divider to be able to control overvoltage.
You can also connect 12V via a voltage divider.
Do not forget to connect the ground between the PSU and the Arduino.
I would use all three measurements to control the power supply.
Save yourself the trouble. A typical ATX PSU has got this built in already and its response to a fault condition will be faster than your arduino circuit.
The only thing that may make sense is an overcurrent protection circuit that limits the current draw of critical or fault-sensitive components that are within the boundaries of the PSU's current rating (which, you guessed it, the PSU will protect for by itself already).