You didn't tell us the power supply make and model. If your PSU DOES NOT COME WITH the necessary adapters or cables to plug all three in, buy a different PSU.

If your PSU COMES WITH all the necessary adapters or cables to plug all three in, that's fine. If your PSU only has the ability to plug into two, buy a different PSU. Have you ever wondered, 'has this question been asked previously'? But, if your PSU doesn't have enough cables and connections available to make that happen, it's safe to use one 8-pin to 8-pin cable and one 8-pin to dual 8-pin cable (thus have two 8-pin connections on the PSU side for three 8-pin connections on the GPU side), as long as your PSU came with the necessary cables to do so. If at all possible, use three separate 8-pin connectors. That's how you can easily know if the PSU manufacturer designed all the other necessary components to withstand such load. Only do it if the PSU came with the necessary cable. Don't use such cables or adapters from aftermarket sources to plug into a PSU which didn't come with such cables in the box. If the PSU is a high quality unit, and if it comes with the wires to plug two 8-pins into the video card for only one 8-pin on the PSU side, you can rest assured that the PSU manufacturer has determined that it will be safe to do so. For example, many Seasonic PSU's have their 8-pin connectors rated for 270 watts instead of the bare minimum 150 watts, which is an increase in 80% more wattage per 8-pin connector or up to 540 watts at the PSU connector end per each 8-pin connection. Many quality PSUs have high-enough quality wires, insulation, pins, and connectors that can withstand much more than the bare minimum ATX specifications. No, it doesn't necessarily need three individual connectors at the PSU

Power supply with 3 8 pin connector