Size 8 Pampers diapers are meant for more chunky toddlers or for those that are bed wetting such as a 5 or 6 or 8 year old child.
So far I haven't seen pampers size 8 diapers in stores or available in the USA but I have seem them on Amazon and ebay.
My nephew wears size 7 diapers and those are still roomy enough for him and he can likely wear those size 7 diapers for awhile yet for his bed wetting.
Size 7 diapers are meant for those children that are above 35 lbs in weight.
My nephew who is 7 years old fits easily into size 7 diapers and wears them for bed wetting or on long road trips where he will fall asleep and wet his pants.
The 7 year old nephew still has some room to grow in the size 7 diapers and they will probably still fit him until he's at least 9 or 10 years old unless he gets bigger in the waist etc.
Some 3 year olds are in size 6 and size 7 diapers while some 3 year olds may still fit in size 5 diapers.
It all depends on the child and the way they are built.
Some children are taller and bigger in the waist than others so the diapers can fit different children at different sizes.