Don't worry - Tamagotchis change at their own rates. But age 3 is actually normal for an adult Tamagotchi - sometimes they even evolve at 2! 3 or 4 is the average age, but the age counter is a bit messed up so anything from 2 to 5 can be considered usual .
Because V1 Tamagotchis get an extra "year" every time they fall asleep, we're all used to having 10 year olds changing into adults, so people think something is wrong when theirs changes at 2 or 3! But V2s and V3s are different, so they change at different ages!