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:
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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