Regardless of the power supply unit, if the card is operated with only 2 or 3 power cables connected, its performance will be very reduced or it will crash. If the power supply unit no longer has any free plugs, a new power supply unit must be purchased, otherwise ask the manufacturer for additional cables. Never use a Y-cable.

2 power cables is correct, you have 2x 6+2 PIN at one end of each cable, i.e. a total of 4x 6+2 PIN and you connect 3 of them to the graphics card and one remains. I hope I was able to help you.