Quick help needed, as I want to order within an hour or so.

I'm online shopping for my twin cousins who I'm about to meet for the first time. They are 2.5 months old. One is bigger than the other.

I decided on dresses and bows (two dif colours) from Baby Gap.

Should I buy them size 0-3? 3-6 months? Or one in 3-6 and one in 6-9 so they'll have 'a' dress for longer time? Or both in 6-9 because babies stay that size longer? Help! (fyi they seem like 'normal' size babies and were not premature).