It is not at all unusual for 3-year-olds (give or take a few months) to have difficulty sharing. My grandson, Patrick, for example, was anything but uncertain in his reluctance to share anything with anyone when he was 3. Why this is typical of this age is anyone's guess, mine being that the toddler is just beginning to grasp the concept of private property. That enables him to exercise a greater degree of control over his immediate environment, including his playmates, while at the same time making a clear assertion of his identity. So, whereas not sharing needs correction, it is not, in and of itself, bad.
Unfortunately, some of these "nonsharers" tend to become aggressive toward other children who try to play with their toys. Needless to say, their aggressive outbursts are not consistent with the Marquis de Queensbury rules. The 3-year-old is undersocialized; therefore, when he strikes out aggressively toward another child, the assault is likely to look distinctly savage to the civilized onlooker.
The first problem - simple, garden variety nonsharing - requires parents and other caregivers who are firm in their expectation that children share and provide the disciplinary structure within which sharing can be learned. One way to do this is to use a kitchen timer to signal when the toys in question have to be exchanged. In most cases, once children have learned to take turns in this fashion, they begin spontaneous sharing in almost no time.
The answer to a 3-year-old who hits, kicks, pinches, and bites is to immediately remove the child from the playgroup, even if it's a group of two, until he is ready to apologize and share. If an aggressive incident occurs twice in the same play session, I'd recommend confining the child to his or her room for the remainder of the day, along with an early bedtime. If it occurs in a playgroup, I'd recommend separating the child from the other children in the group for the remainder of the session. To those who would say that separating a child from the group from, say, mid-morning , until his parent picks him up at the end of the day is too much, I respond thusly: If one wants the aggression to stop - which should be everyone's aim- then the consequence must be memorable. It must create a strong mental imprint that will begin to inhibit aggressive outbursts.
Meanwhile, adults should definitely use the opportunity to counsel the child on nonviolent means of handling similar future situations.
In the final analysis, however, nipping behavior of this sort in the proverbial bud requires parents who absolutely will not tolerate it.
Copyright 2019, John K. Rosemond