I think it depends on the person ! I'm not in any big hurry to get on a 3 yr old, but if he tells me he's ready, then we're good to go - it's about lstening to the horse. I'm very light on my two and there's a HUGE amount of good fun you can have at a walk. However, I use that year not to develop particular exercises or movements, but to establish the contract between you and the horse. Set up the 'agreement' you have with him.
For example: suppose I were teaching a class of teenage (human) children - I would want to tell them :
'it's not about the amount of time you spend in this lesson, it's about where we get to'
and 'I can't teach you anything, if you don't want to try'
So, to motivate teenage children - I'd scrap the whole idea of 'an hour's lesson' or whatever, and say 'you can go home as soon as we get to point X'
Alas, I can't explain to the horse what point X is that I have decided to get us to that day! No problem - instead, I can have an idea in mind of where I want to get to - but I have to be prepared to throw it out at a moment's notice.
The contract I am aiming to establish with the horse is :
1. 'you will feel pressure but it will never be severe'
2. 'as long as you try, the pressure will never increase'
3. 'as soon as you get the right answer, all pressure will go away immediately'
4. 'if you make an extra-special effort at any time, you will be rewarded big-time'
point 4 is really important. Well, they all are. Right from day 1, with your ground work - apply very very light pressure, and continue to do so all the while the horse tries to find the right answer for you. If he's flipping you off, increase the pressure. Otherwise, if he's trying and getting wrong answers, be very careful not to up the pressure. if you go more than 1 1/2 minutes without something nearer the right answer to reward, then change the question. Point no. 3 is important : focus on getting your release timing good. No, not good - great. A great horseman is one who reads the horse so well that they release the pressure before you've even noticed the horse's correct try. He noticed the horse's correct thought of a try !
Point 4 is, I think, the one single contribution I would want to make to horse training, It's been said a million times before, but I would like to bring it forward more into people's minds. We're so used to getting our 'value for money' .. making the most of our time. We know we need to get the horse fit, and we know we've paid to hire the arena (or the instructor) for the hour, or we have an hour of daylight left to make the most of ... so we want to use that whole hour. Hogwash. Go out there, warm up, do some stuff, but if your horse makes a really great effort at anything - even if it is leading up to the arena gate, even if it is putting his head into the bridle before you ever get on - feel it in your heart, if you KNOW deep in your heart that was a great try from him.. then even if your instructor is watching, even if it's 2 minutes into the ride, reward him, pet him and put him out with his buddies.
You lose a lot of good hours riding and training that way, and your horse stays fat and flabby til he's 4 ... but as a a 4 yr old, OMG you won't believe (and neither will everybody else watching you) what a fantastically willing horse you have. OMG, they will say, "he's all try" ! You'll put your leg on his ribcage for the first time and ask him to leg-yield, and he'll just trot sideways for you straight across the diagonal OMG everyone will say - how long have you practiced that ? ( errr, that was the first time - he just wants to please me, and he knows how to yield to pressure!) - he's constantly alert for your every request - his paces are active and his focus is on you - because he's always looking for that one big try that he gets his big reward.
The 3 yr old year is one of restraint. Of setting him up for success in his subsequent career. Of enabling him to be as soft, willing and responsive as he can be.
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