it is easy (or easier) if the child is exposed to a foreign language everyday and they will pick it up from there, but to put a 3 year old in a language class is infringing on the child's human right!
the progression of learning a foreign language without practice is very slow, even for an adult.
when i was at school, there was a HK kid. all he does is mix with other HK kids, watch HK TV, etc. so his english never improve. during a physic lesson, we were learning about magnets. and one word that made the whole class laugh was when he said magnetic flux density!