The latest update to Cura (3.6.0 at time of writing) adding a printer configuration specifically for the Ender 3.
The Start G-code script now includes the following:
; Ender 3 Custom Start G-code
G28 ; Home all axes
G92 E0 ; Reset Extruder
G1 Z2.0 F3000 ; Move Z Axis up little to prevent scratching of Heat Bed
G1 X0.1 Y20 Z0.3 F5000.0 ; Move to start position
G1 X0.1 Y200.0 Z0.3 F1500.0 E15 ; Draw the first line
G1 X0.4 Y200.0 Z0.3 F5000.0 ; Move to side a little
G1 X0.4 Y20 Z0.3 F1500.0 E30 ; Draw the second line
G92 E0 ; Reset Extruder
G1 Z2.0 F3000 ; Move Z Axis up little to prevent scratching of Heat Bed
; End of custom start GCode
This prints 2 initial lines, right along the left edge of the print bed. This solves the priming issue completely and results in a perfect initial border on the rafts.
This would indicate this priming issue is a known problem with the Ender 3.