The size of diaper that a 3 year old child wears is between a size 5 to size 6 diaper depending on their weight.
Some 3 year olds are skinnier than others and some 3 year olds are heavier in weight than others and have a bigger waist.
But for an average size 3 year old child they will usually be in a size 5 or 6 diaper.
If you get a size 6 diaper the 3 year old should most certainly be able to fit in it and if you buy a size 7 diaper for the 3 year old they should easily fit into the size 7 diaper with more room to grow.
Size 6 and size 7 diapers can usually fit a 6, 7, 8 and even a 10 or 13 year old with ease.
So it would be better to get a bit larger diaper especially if you know the 3 year old child will still be needing to wear them longer.