This will all be fine - give it some time. You can imagine how disconcerting it can be to a young child to suddenly have a sibling sort of disappear for stretches of time. This is hard for a young child to understand, primarily because the young child has no framework for what has happened. Be sure that the daycare setting sets limits around the aggressive behavior. Otherwise, be alert to questions your three-year-old might have about his brother and his whereabouts. Have you shown him the
schoolPreschooler development
Preschooler test
Preschooler test or procedure preparation
School age child development
School age test or procedure preparation
School-age children development where his brother attends? If you haven't already done so, let him come along a couple of times so he can see where his brother is. If you've already done these things, you've pretty much done what you can. Be patient - he's just reacting to a significant change in the environment, and he'll adapt.