The different variants of the power supply have a different cause.
The 6-pin PCI-E connector is specified for 75 W.
So 2 of them brings 150 W.
But you will need about 400 W for the miner.
The cables from the power supply can thereby be warm.

3 connectors brings 225 W.
So the cables from the power supply is less stress in this case, since the load is distributed across multiple cables.
In my opinion it is not another reason for the 3rd connector .