This is an excellent question. I would assume that they mean you should allow 3mm extra on each side, for a total of 6 extra millimetres. So your entire file would be 303x426mm.
This is extremely unlikely, but to be safe, you should make sure that nothing you don't wish to lose is closer than 6mm to the edge.
They haven't given you enough information yet. Have you read this?
What software are you using to design this file? If Photoshop, please also read this.