Ground driving them at this age is a great way to start...because, ground driving is so important for the big foundation that a great driving horse needs.
But... Also, it does depend on the actual horse himself...some just aren't ready mentally or physically...and, you just need them to grow up more.
I think its a good idea as well to have the Vet check them to see if there knees are closed and, ready to take the stress of a working horse. They are no different then the big boys...knees are knees. I would rather wait a bit for them to mature than have problems down the road.
I don't want to upset anyone...but, I see alot of people just start these 3 year olds- just because they are three...IMO...alot of things factor into.. is my young horse ready.
I guess the moral of this story would be just take your time...and, learn to read your horse...he will tell you very quickly if he is ready or not. If he is...by all means start...if he is not...give him the time he needs to grow into a ready to learn successful adult.
You may also, get into the process after sailing along in his training and, then come to a stand still...thats his way of saying...give me some time to absorb all the information...he may just need a litte break...before you continue on.
Make it fun and interesting for him...and, you will be a successful team!!
It sounds though your youngster is saying...lets get started. Just take your time... Good Luck...and, have lots of fun...driving is just the best!! I am happy for you