3.3V digital signal as input to Arduino mega 2560
You just need to refer to the ATmega 2560 datasheet

Page 355, electrical characteristics




  Notes:
  1."Max" means the highest value where the pin is guaranteed to be read as low.
  2."Min" means the lowest value where the pin is guaranteed to be read as high.


For Vcc=5V:

The LOW level is below  \$ 0.3 \times V_{cc} = 0.3 \times 5V = 1.5V\$ 

The HIGH level is above \$ 0.6 \times V_{cc} = 0.6 \times 5V = 3V \$ 

If your sensor (or whatever the source of the voltage is) can actually deliver 3.3V then you are fine, but if the voltage is actually close to 3V then you'll be in trouble.