Young growing horses have higher nutrient requirements than the “same” adult horse. The requirements are similar to an adult in moderate to hard work. This means that most adult feeds are not formulated well for those nutritional needs.
Low sugar Growth feeds are best if the calories are needed. Ration balancers have higher feeding rates for youngsters than for adults to take into account those increased nutrient needs, so in that category, the same feed can work for an adult and, say, a yearling, but the adult might get just 1lb and the yearling 3.
I have used both a growth feed (TC Growth, pretty much the lowest NSC growth feed at the time) and a ration balancer (Progressive Grass Advantage, Tribute Essential K, and TC 30) based on the calorie needs.
Feeding less of an adult feed may well be short-changing needed nutrition. Will it make a difference? It all depends, and the effects may take years to show up. It’s less likely to matter for a 3yo than a yearling or younger.