Preschoolers enjoy the company of other children and interact with them instead of playing side by side like they did as toddlers. This shift happens naturally for 3-year-olds because they spend the majority of their playtime in fantasy activity, which tends to be more cooperative than play that focuses on toys or games. As they learn to take turns, share, and create new games together, children start developing important social skills such as paying attention, communicating (through actions and expressions as well as words), and responding to each other appropriately.
Shelov, S. P. (Editor-in-Chief). (2004). Caring for Your Baby and Young Child: Birth to Age 5. The American Association of Pediatrics. Revised edition. New York: Bantam Books.