Your 3 year old toddler is not choosing to play with other children rather than just being near them. Sharing and taking turns comes a bit more naturally now, and they are beginning to show empathy – ie they can see things from someone else’s point of view. More about your 3 year old here.
From the age of 3 children can start to understand why they shouldn’t behave in certain ways, and it’s never too early to start setting boundaries for rules and behaviour. Discipline techniques like the naughty step, reward charts and praising good behaviour are all useful tools at this age.