Teaching 10 three-year olds is definitely a two person job. Having a child sith Down syndrome in your class should be a pleasant experience for all. You may also need another helper for the other nine so that you can give full attention for short periods of time to each child. If the child with Downs could reach a juice pitcher it is too low. Keep such things out of reach of all three year olds.
Challenged children can learn and interact just like other children.
You might also start seeing the child in a more positive light and plan your lesson accordingly. Use lots of gestures, some sign language, flannel boards, pictures, songs, etc. All the kids will love it. It is filled with wonderful, positive ways to nurture children with special needs and contains a chapter especially for the church. I would also encourage you to relax and enjoy your young student.