The higher order value of a three-packer gets it picked up by Customs - so VAT & R. Mail handling charge eat up any savings. Just did a quick calculation and I see there's practically no cost difference between ordering two lots of three, or three lots of two. The advantage with the three lots of two route is speed of delivery, as having to pay the VAT-man (which can be done via the R. Mail website with a credit card) adds another 4 or 5 days to total transit time.
I suppose you could save a few quid and buy in bulk: get a couple of the triple packs at a time and take the VAT charge on the chin. But it's not really going to save you that much....
Slarti