OnePlus 3 & Oneplus 5 viewport size for chrome Emulated Devices
I was searching for the same thing and I found this: https://www.mydevice.io/
OnePlus 5: 455x809, pixel ratio 2.375, user agent Mozilla/5.0 (Linux; Android 8.1.0; Build/OPM1.171019.011) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/66.0.3359.158 Mobile Safari/537.36
I don't have an OnePlus 3 and I didn't found anything related.
The OnePlus 3 uses a 346x535. Not a standard one as far as I can see, with all the consequences it brings in.