what does bring to the friendship of these three children

He brings to them a different perspective of the outside world. Their world is solely in Maycomb County, and observing the childrens first conversation, the reader can see the curiosity of the outside world in both Jem's and Scout's voices. He also brings another life style, meaning another way of living. At first they do not realize Dill's living situation, but after they understand his situation and their mind is broadened in how other people live and they are able to appreciate their own lives better. And offcourse Dill provided them with a friend, since both Scout and Jem always looked forward to spending their summers with Dill.