G-Code files from Cura stock profile for Ender 3 to help you test your bed leveling and also Z-offset files to calibrate the Z offset that gives you the perfect first 0.2 mm layer(The squares are 0.2mm high).
On each Z offset model will be written the negative offset that it was printed, look closely what looks to be the perfect layer and type the Z offset in you slicer (in cure download Z offset plugin from their market place, on Prusa Slicer the Z offset setting is under printer settings.)

Firstly:
Print the CE3_Z_BedLeveling.gcode and look the difference between the squares and rotate the bed leveling knobs to make all of them to look similar, even if all of them will be split lines- the next step will take care of the Z offset.
As you can see from my image- my bed on the left side was too low(split lines).
Secondly:
Start with CE3_Z_offset_0.gcode and go up-to CE3_Z_offset-1.gcode until your print has perfect layer look.
Finally:
Update your slicer with the Z offset that was the best for you, for my printer- I found out that Z -0.075mm was the best, so I set this in the slicer(Z offset -0.075) and now I can get perfect first layer :-).