It is important to know that Intel® Extreme Tuning Utility (XTU) was designed to work on Intel® motherboards only, not on Original Equipment Manufacturer devices or other system brands devices.
If you still want to use the utility, you may want to use an older version of the software.
If it still not working, you may check if you can restore your computer to the last working state by choosing “System Restore” once you are in Windows*.
Additionally, I suggest confirming with Microsoft* how to under-volt your BIOS probably by flashing stock BIOS.
You may visit Download Surface software, firmware, and drivers from Official Microsoft Download Center for firmware downloads in order to restore your Surface 3 to OEM specifications.