Hello Vicente,
Firstly, when you use windows API to access NFC reader, no errors occured on API, so it means NXP-NCI(windows API for NFC) has been supported in windows10 iot SDK for Raspberry Pi 3.
But the value API retruned was no readers, it seemed that PN7120 driver was not installed in windows10 IOT core by default, so you can try to re-install PN7120 driver on your platform, the following link is driver files for PN7120:
PN7120|NXP
---- PN71x0 on RPI winIoT driver installation files (REV 1.1)
This package contains the files for the driver to run the NFC Controller (PN7150 or PN7120) on Raspberry Pi in Windows IoT.
ZIP
- (7.3 MB)
- PN71x0 on RPI winIoT driver installation files
- 5/30/2016
In addition, you can also refer to AN11646 document: http://www.nxp.com/documents/application_note/AN11646.pdf
Best Regards,
Weidong