A 3v XHP50 can be put on a 6v XHP50 mcpcb yes (and will still only be 3v) as a 6v setup is still just a standard pad layout but a 3v led CAN NOT be ran on a 12v mcpcb (that would instantly break it as you’d be shorting cathode to anode).

If you’re using a sinkpad capable of either/or with a 3v led set it up as 6v with the jumpers bridged, NOT the 12v method with the resistor. This goes for 3v XHP50 or XM-L / sst-40 as well.