He takes a bite, drops a third of his mouthful..then chews. I immediately thought "tooth issue"..but I'm being told since he's only 3 its because he doesn't have back teeth it is harder for him to chew.
Horses usually lose all their foal-teeth around two and three, so it could just be that yes, he's missing some teeth, and until they grow in, he's probably going to find it awkward and maybe a bit painful to eat.
However, there could be other problems, too, maybe an ulcer in the mouth or maybe he just accidentally bit his tongue. The only way to know what the real problem is is to have an equine dentist or a vet look in his mouth.