Alfalfa is good for growing rabbits as it is high in calcium and protein so helps their growing bones and muscles. However it is too high in calcium and protein for adult rabbits so should only be given as a very occasional treat, if at all, to an adult rabbit. At three months old they can have alfalfa for a while yet, but I would also feed what will become their usual hay, as it is good for them to get used to the taste and get into the hay habit while they are young!