They are all 5V as the Mega2560 uses 5 V logic. However, there is a 3V3 pin at the bottom, second pin from the left, that you could use:
However, unfortunately, the pin isn't protruding, so you'll have to make an ugly solder link to the top of the pin.
As an aside, the Iteaduino MEGA 2560 works at 3V3 but unfortunately the Mega shield that you have won't fit it.
The Uno equivalent of your shield will fit it though:
Greenonline
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