I can’t help you pick one but I can at least tell you their strengths and weaknesses to help you choose which one you want.

All three of these are great for gaming..

The ROG phone is the most famous gaming phone of these three, it has a good cooling system, a good OS and it has good performance stats, but , ROG phone doesn’t have 5G and it is expensive.

Red Magic 5G is a new phone from nubia and it is a good phone that has good specs and good gaming performance, it also has 5G network, however the OS isn't great and they may not update the software often.

Black Shark 3 has the same CPU and GPU as theredmagic and also has5G, but the black shark 3 has a lower refresh rate screen, and you cannot buy it yet until May 8th.

For Razer phone2, I think it is good for you if you are a Razer fan. If not, plz do not buy it cos it just won't keep up with the games you want to play..