Did you completely remove the display itself, too? If you did not:
Two things are always recommendable with screen issues:
First, check if there are gaps in the seam between display and core module. If that is the case, close them with gentle pressure. (see below, iFixit instructions Step 9, black text).
If that doesn’t help, removing the display altogether and putting it back on might help, as described here:
If all of that doesn’t help, I would support Martin’s suggestions and test someone else’s FP3 screen on your FP3 (and/or vice versa). Check if there’s a Fairphone Angel in your area if you don’t know any other FP3 user nearby.