Cantering undersaddle will probably be easier in him than lungeing him at a canter. At three, in my opinion, a horse is mature enough physically and mentally that they can handle a little cantering. I don't lunge horses much so I can't give you advice about slowing down the canter that is specifically for lungeing but I will say that if you can control the speed of the walk and trot then you will be able to control the speed at the canter. I don't mean just getting a slow trot but being able to speed it up and slow it down. I find it much easier to control a horse when I have my legs on either side and a hand on each rein rather than standing several feet away with a whip and a line.
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