A 3mm blue vein may be too thick for laser treatment. With that sort of volume of blood within a vessel, it’s difficult for these lasers to be effective. Other methods, such as sclerotherapy and vein stripping, are more appropriate for veins of this size.
For viens that are 2mm or less the pulsed dye laser is recommended, however the Nd:YAG laser has been used for this as well.