BGv3 are just a newer version of the BGv2, the main differences are 1) extra inch of PUL on the inside of the nappy, and 2) stronger velcro/aplix which stays sticky longer and doesn't go all curly. Also they come with a large and small microfibre insert, instead of just the 1 that the v2 came with.
oh and btw, the Greenkids pockets are also onesize but without the poppers to adjust the height - there's a special way of folding them down (which I haven't tried yet, as only used them on bigger babies). But have to say, we love GKs for daytime - that's about all Clement wears during the day - they're fantastic. But I didn't have success with them at night because his vest was picking up dampness from the top/front of the nappy, esp when he rolled to tummy.