Actually, it is not "a must" to change the formula from no.2 to no.3 at one year old for your baby.
I know that some moms still allow their baby to drink no.2 or even no.1 till 2 or 3 years old. As the nutrition value for no.2 and 3 are not much different. Even tho it always highlight the 'high protein' part in no.3 or no.4 formula. However, doctors said, the main source of protein should be absorbed from solid food instead of formula.
Just my opinion....
Of course, you can try a bit longer for your baby to adapt to the new taste of No.3... some babies need more time when changing formula. Don't give up.