The 3000MHz is the rated speed the RAM can be set to. It doesn't mean it can't go above this however, however the RAM is tested and rated as stable up to those speeds.
Your MB will have a native frequency, so a motherboard may have RAM frequency capabilities of 2400, 2666, 3000 (OC), 3200 (OC). The OC basically means you need to overclock in order to operate at that RAM frequency, but you can run 2400 or 2666 without OC.
2666 RAM basically means its rated officially of being able to operate up to 2666 MHz, but it doesn't mean you can't go above that, just means that if you do, you're risking more instability and it's not "officially" supported. So the idea would be you get RAM rated at the speed you want to run at. I.e. if you want to run at 3200, get 3200 RAM.
The PCX XXXX is a description of the transfer rates and type of RAM. But is basically describing the transfer rate. All you really have to do is make sure it is DDR4 and supported by your MB.