this could be quite simple
Here is what you need to do:
> Make an OpAmp comparator which has a reference of 3V on the Inverting I/P and VCC of 3.3V and Output of Comparator connected to the Microcontroller I/P
> Give your I/P voltage to the Non-inverting Terminal of an OPAMP Comparator
> Now whenever the I/P Voltage is less than you get a 0 Volts at the output of the comparator
> When the I/P voltage exceeds 3V the comparator Output goes to 3.3V
makes sense? no idea of what a comparator,opamp is?