Depending on when they were born, it'll be when they reach 3 years or 4 years, it won't always be the exact day they age up to 3 years, often they'll still be growing for another 1-12 days, in some cases more than that

An example of 4 years would be if they're born in winter, as they age up a year before it's been 12 days(a full game year). This is why you'll often see 2 year old wolves that weigh 1 pound.