NO ..3mm is the equiv of 1/8 and 6mm is the equiv of 1/4
They are not an EXACT conversion but a close equivalent.

BUT ..If your refering to pneumatic / hydraulic port sizes it's more complicated... the port size refers to the inside dia of the pipe that the thread is ON and has no numerical relationship at all to the hole size.

I did give you a link to my threads page.

Here's a helpful bit of software for you for normal conversions.

http://joshmadison.com/software/convert-for-windows/