I designed this case for my OnePlus 3. I have been hesitant to upload it since it is a pretty old phone by now, but some of you may still be using it as I am.
This model is designed to be printed with TPU or other flexible filaments. It will not work with PLA or ABS.
This model can be printed without support, but it can give slightly better results with support. Keep in mind TPU supports are more difficult to remove than PLA ones, so make sure you configure your supports correctly if you choose to use them.
The walls on this case are 3mm thick. I made it this way because I wanted to print with 2 outer perimiters and 20% infill to give it a bit of springyness and drop resistance. You're free to print it however you like, but that is what worked for me. I also printed it with no bottom layers so that the infill of the model is visible and gives it a cool texture. I used gyroid infil, which is quite grippy. Have fun experimenting.
I also have the original model available on Onshape for modification at the link below: