Bed Leveling
To test the bed level you can use this Ender 3 Bed Test or this Ender 3 Full Bed Test.
Easy Method:
3D Printing Essentials: The Best Way to Level a 3D Printer's Print Bed
Advanced Method:
Use a Feeler Gauge (or alternate link) to gauge to set the Z offset.
With a 0.4mm nozzle it is recommended to gauge 0.25 0.20mm using a 0.4mm First Layer Height.
With a 0.6mm nozzle it is recommended to gauge 0.35mm.
Doing a Y Axis Roller Adjustment might help with more consistent Bed Leveling.
Also see Bed Leveling Testing.
Also see First Layer Settings.
Make sure to Preheat the Nozzle Temperature, before Tramming the bed.
An improper Trammed Bed could lead to poor First Layer Adhesion.
Supporting Documentation
Videos:
3D Printing Essentials: The Best Way to Level a 3D Printer's Print Bed
Shop Talk - Stop Leveling Your Bed and Start Tramming Your Machine!