What is 3W Rule of PCB design?

This rule applies to the calculation of line width and distance of 
coupled microstrip or coupled striplines. This are used for differential 
highfrequency signals (for ex. HDMI, SATA, PCIe).

To ensure a good coupling between the positive and negative line, you 
shall follow this 3W rule.

Christophz wrote:
> To ensure a good coupling between the positive and negative line, you
> shall follow this 3W rule.

No.
To ensure a minimal coupling between two separate single ended lines, 
you should have at least 3w distance between these lines to minimze 
cross talk. It depends on the signals on these lines, especially the 
rise/fall time.
If you have enough PCB space and you can spend more than 3w, then more 
than 70% of the line electric fields will not interfere with each other.

On differential lines a coupling between both lines is desired. In this 
case, the distance normally is less than d=3w to get a sufficient 
coupling. The correct value is a result of the impedance calculation. 
The result depends beside the distance d of the PCB thickness (distance 
to GND layer), the desired Z-value, the PCB material and a little bit of 
the line thickness.

If you have several differential line pairs near by near, the distance 
between these pairs should be also at least 3w to avoid unwanted 
coupling.