I have vivid memories of my brother (Dr Bob) and I tearing around the slopes at a very young age. I think he was 3-4 and I was 5-6. We both had a lot of fun and I don't think it was too young. Those were the days of leather ski boots and wool gloves (with frozen fingers inside!) An injury at age three is not any different than at age 5 if that's what you are worryied about. Luckily, equipment is much safer and warmer now. A helmet is an absolute necessity at this age (actually at any age if you think about it).