Are you sure he only does it around you? I mean, I swear used to Alli see me coming and decide to windsuck out of spite
because I always tried to stop her, but I also know from the land owner she used to do it between visits too. I'm not saying he's winducking btw.
He could be biting the bars out of boredom, anxiousness about getting out to work or go in the field-hence why he does it when you are around-, stress (anything new to upset him?), mineral deficiencies.... Loads can cause vices like this. Try putting a mineral lick in his stall, preferably one of the hanging ones so he has to work for it and keep his mind busy. This helps with the possibility of deficiencies and boredom. Put something like cribox (Cribox & Crib Biting Prevention
) which tastes really mank (honestly
, don't get it on your fingers then try to eat something!!!!) on the bars to deter him and tell him off for doing it.
Whats his feeding regime like? Is he being fed grains or mollassesed feeds? Because these high starch, sugar and protein feeds can cause digestion problems as the horses stomach is not designed to break down these types of nutrients so the wrong type of gut bacteria thrive. Also, does he constantly graze on grass or hay? The stomach lining constantly secretes acid and this is only neutralised by saliva when the horse is chewing. A build up in acidity can lead to stomach ulcers, which cause pain. (Stomach ulcers can also be caused by stress)
Also, three years old seems quite young to be learning to do whats in those videos. He isn't fully physically developed yet so ridden work should be low impact stuff like hacking and light schooling so he can find his balance with a rider, and so his muscles, bones and joints can continue to grow and develop without injury. Some horses don't fully develop until they are 6 or 7, but generally 4 is seen as acceptable to start proper training. I don't know how far in to the training he is, but could he be stressed from mental or physical overload?
Hope this helps