Our three-year-old still co-sleeps with us. Is it time for her to sleep in another room?

Yes, i would think 3 is a good age to start making them go to bed on their own. It doesn't have to be a direct change. Try and ease them into sleeping on their own. They may be afraid of sleeping alone due to many reasons. Start of by sleeping in their bed with them till they fall asleep before leaving. Eventually they will get used to it.