Is the VIN on the N3 5v-12v or 7.5v-12v? According to the documentation here it states 5v-12v, however directly on my N3 Wifi, the silkscreen states 7.5v-12v. Also looking at the schematic it uses a TLV1117-50 which seems to have a minimum voltage of 6.4v.
There are two ways to supply power to the Netduino range of boards:
If you are using the Vin pin on the headers then you should be using a regulated 5V power supply.
The power barrels on all of the Netduino silkscreens are marked with a lower voltage of 7.5V. I would stick with that as the lower voltage if using the power barrel. LDO regulators usually like a bit of headroom.
And I forgot, USB can be used as a power source if 5V is applied across the power lines.