It has been verified that 11.3 causes issues with iPhone 8 3rd party screens.
http://bgr.com/2018/04/10/iphone-8-bug-i...
1) Connect to a computer and backup the phone completely in iTunes.
2) Download the 11.2.6 firmware from here.
3) Put your phone in DFU mode
4) connect to itunes, and it will as you to restore
• on a PC, hold SHIFT + click the restore button, and select the 11.2.6 firmware you downloaded
• on a Mac, hold OPTION + click the restore button, and select the 11.2.6 firmware you downloaded
Restore the backup you created in step 1.
Now wait until a new iOS comes out that is newer than 11.3.0 (i.e. 11.3.1 or higher).