A simple calibration for fine-tuning your three extruders offsets. On the Y-axis: The distance from the first extrude bottom edge there should be 10 mm to the second extrude bottom edge. And to the third there should be 20 mm. On the Z-axis: The distance from the first extrude left edge there should be 10 mm to the second extrude left edge. And to the third there should be 20 mm. Print one layer or more, pause, measure, adjust offset, save and restart print until you have a perfect offset.