In recent years, interest has increased in preschool programs that promote low-income children's early learning and development. Although research in this area has focused mostly on 4-year-olds, a growing number of 3-year-olds attend publicly funded preschool. In fact, in Head Start--a federally funded early childhood education program--the percentage of 3-year-olds rose from 24 percent in 1980 to 40 percent in 2013. Increased attention has also been focused on identifying promising strategies to enhance young children's social, emotional, and behavioral development. A number of mostly small-scale studies demonstrate that classroom-based approaches can improve these outcomes for 4-year-old children. However, a notable gap in the evidence base is whether the benefits of these social-emotional strategies can extend to younger children in the classroom. This report presents exploratory impact findings for 3-year-old children from the Head Start CARES (Classroom-based Approaches and Resources for Emotion and Social skill promotion) demonstration. The Head Start CARES demonstration was a large-scale randomized controlled trial implemented in Head Start centers across the country. The demonstration tested the effects of three classroom-based approaches that each had a distinct theory, developed by the Head Start CARES research team, of how to improve children's social-emotional competencies. Called "enhancements" because they were intended to complement and to enrich existing Head Start classroom practices, the three approaches are The Incredible Years Teacher Training Program, or "The Incredible Years"; Preschool PATHS (Promoting Alternative Thinking Strategies), or "PATHS"; and a one-year version of Tools of the Mind focused on play, or "Tools of the Mind--Play." The demonstration was conceived and sponsored by the Office of Head Start and the Office of Planning, Research and Evaluation in the Administration for Children and Families, U.S. Department of Health and Human Services. MDRC, a nonprofit, nonpartisan education and social policy research organization, conducted the demonstration in collaboration with MEF Associates and several academic partners. [For the full report, see ED558517.]