Summary

An 100% printable phone arm!
It weighs about 0.25 kg and is easy to assemble.
Each arm has a working length of 150 mm, so the 3 of them provide a total length of 450mm.

There are 10 different parts.
All of them were printed with 30% infill in PLA, apart from the "Base Screw Pad", which was printed with 50% infill in a flexible material for better grip (TreeD Flexmark 9). It is a hard TPU, which is easily printed without any modifications in Creality Ender 3.
60% infill may be a better option for your screws.

Below you can find some screenshots from Cura, to see how I suggest you to print the parts.

The "Arm End" is printed separately from the "Arm" in order to avoid warping in the thread section. It needs some force to put "Arm End" into the "Arm". If it is not possible to fit, try to sand the base of the outer cylindrical surface of the "Arm End".

The first time (or two) that you are going to use the screws, there is going to be some resistance but after that they should work properly.