When talking about sharing with 2 and 3 year olds it is important to decide if you are talking about sharing or taking turns. Turn taking is easy to teach and in most cases that is what parents mean when they say sharing. The question asked this time had to do with parents trying to get their children to share with another child.
1. Be sure you mean sharing and not taking turns.
2. if it is sharing then both end up with something at the same time so it often needs a little adult involvement initially.
3. If it is taking turns then involve the other parent by saying we will take turns taking turns. It doesn't matter who goes first.
Set the children up for experiences.
Word of caution: Do not force a child to share. It will not teach them sharing skills, but it will teach that their will can be overriden by someone bigger and they will apply that with others who are smaller than they are. If a child has a favoite toy then they should not be expected to give it up.