Dressing up will enhance your child's fantasy play, but lots of fancy costumes, although fun, are not really necessary for an imaginative preschooler. Props are much more essential to her imaginative play. A fully equipped play kitchen, for example, or just some old pots and pans and cooking utensils that you seldom use for "real" cooking anymore, will probably get a lot of use this year.
You can use the eagerness to play new roles to help your child express emotions. Take turns acting out the parts of a tired old man, a hungry baby, or a happy puppy. Roleplaying that specifically encourages emotional expression helps your child use her body to better understand her own feelings (and the feelings of others). Through play, emotions can become more familiar—and safer.
Your three-year-old will engage in more and more elaborate imaginative play. She will whip up entire make-believe scenarios with characters and events in abundance. Tricycles become cars, ambulances, motorcycles, and fire engines—complete with the appropriate noises. Simple cardboard boxes become cars, boats, trains, houses, tunnels, caves, puppet theaters, and castles. She will love it if you help her construct tents and playhouses by draping blankets over chairs or tables.
When your child gets together with one or more three-year-olds, chances are that they'll spend at least some of their time playing house: whipping up meals, putting the "baby" to bed, and so on. Playing house allows every child to play roles that they've observed a lot over the years. It also gives them a chance to rehearse social interaction in a cooperative way, practice that will enhance their building of "real" friendships outside the playhouse.
In her play acting, your preschooler will imitate adult behavior, but you will notice a difference between this and her fantasy play at age two. Your child is no longer merely mimicking adults, but rather role-playing: inhabiting a persona and making it real. Play acting now is not just a matter of having the right props (although that is important), but of assuming the right attitude and saying the right words.
Most of your child's make-believe games will not require you to participate (or even listen). Indeed, it will probably expand your child's imagination more if you allow her to make up her own scenarios rather than offering your input. Let your preschooler create her own private world. If your child invites you to play a role, by all means join in. But take the role assigned to you and let your child control the unfolding of the plot.
Imaginative play also can help your preschooler sort out various anxieties or cope with approaching (or past) events that worry her. You may notice your child playing a lot of doctor games leading up to (or following) a stay in the hospital, for example. Or she may play more at being a parent in the wake of a new baby's arrival.
A word about war games: Most preschoolers do play such games no matter how much their parents try to dissuade them. By all means don't buy toy guns or war toys if they make you uncomfortable. But it's hard to get away from the violence in our society and culture. So don't be surprised if you see your child using a stick to "shoot" or "stab" or engage in "swordplay." Instead of wasting your energy in an attempt to ban certain games or toys, concentrate on teaching your child the value of nonviolence in reality. (Yes, your child does recognize a difference between fantasy and reality.)