Any video card you can buy in theory is not supposed to consume more than 300w - that's the limit of the pci-e standard...
Vega64 consumes up to 295 watts but for very short periods of time of a few tens of milliseconds or something like that, it may consumes a bit more.
It takes around 50-70 watts through the pci-e slot and the rest of around 250 watts are taken through the additional pci-e 8 pin connectors.
Each pci-e 8pin connector is rated conservatively at 150 watts, so even with two connectors the video card would be able to take those 250 watts (approx.) from two pci-e 8 connectors.
Now let's get a bit more technical.
You have voltage and current ... voltage x current gives you power which is measured in watts.
The limitations when it comes to transferring power to a video card are actually due to the contact resistance of the pins inside the connectors, and the resistance of the wires themselves.
The pins inside the connectors are rated for 9A of current, and the wires are rated for around 12 to 16A depending on thickness (less for standard AWG18 and more for the thicker AWG18)
In each pci-e 8pin connector, there's 3 pairs of wires transferring 12v to the video card, the other two pins are not used to transfer power. So, through each pci-e 8 pin, if we want to respect the limitations of the metal pins inside the connectors, we can only transfer 3 pairs x 9A per contact x 12v = 324 watts.
Because this value is so close to the power consumption of the video card ( ~ 250 watts) and because the pci-e standard limits the maximum power to 150 watts, you have two connectors on the video card, so now the video card can get power through 6 pairs of wires ... so the video card can safely receive up to 6 pairs x 9A per contact x 12v = 648 watts.
However, a lot of power supplies use modular connectors and have a cable with two pci-e 8 pin connectors.
On cheapest power supplies, the modular connector on the power supply case is a simple 2 by 8 connector, and you only have 3 pairs of wires (12v and ground) going to the two pci-e 8pin connectors, so even through individually each pci-e 8pin connector could do up to 300 watts, both in total are limited by the modular connector on the power supply case, which is limited to 324 watts (3 pairs x 9A x 12v)
There are some better power supplies which use 4 pairs of wires and route 2 pairs to each pci-e 8pin connector and connect 2 power pins to one wire at the connector, and this means in total both pci-e 8pin connectors are limited to 4 pairs x 9A x 12v = ~ 430 watts
And the best power supplies out there, will actually have a connector on the case with 6 pairs of wires, and each pci-e 8 pin connector has direct connection to the power supply case, and then each pci-e 8pin connector would be limited to the case connector limitation of 324 watts.
As the video card manufacturers can't know how the modular connector on the power supply is wired (3 pairs, 4 pairs or 6 pairs), you will often see the recommendation to use two separate strips of pci-e 8pin connectors if available. This basically guarantees that between the video card and the power supply, you have 6 pairs of wires and therefore the video card can take up to 6 pairs x 9A x 12v = 650 watts.
With Vega64, with some simple overclocking, you can get it to consume 400w, maybe even more. With liquid nitrogen and other sub zero cooling methods, you can probably get it to go over 500 watts.
My guess is they added the third pci-e 8pin connector for those people and to force you in a way to use two separate strips of pci-e connectors.
So... you can use a single strip of pci-e 8pin connectors and it will be fine at stock clocks, and with just two connectors filled.
It would be better to use two separate cable strips with pci-e connectors, to reduce load on the pins on the modular connector on the power supply and to reduce the losses in the wires.
And as long as you used two separate cable strips, you may as well plug the second pci-e connector from one strip into the third pci-e 8 pin connector on the video card. It won't do anything for you when at stock frequencies and even when overclocking a bit, but it also can't hurt.