What are some examples of how Harriet Wheelwright, Tabitha Wheelwright, and Dan Needham impact the lives of both Johnny and Owen?
The three characters you mention in your question are the three most influential adults in the boys' lives. The story couldn't happen without the brief appearance of Tabitha Wheelwright. John's mother is the character that unites the boys in the first place. In chapter 1 John says that the reason he has faith in God is because of Owen and that all starts when Owen accidentally kills Tabitha by hitting a baseball that hits her squarely in the head. Owen feels so guilty he spends the rest of the novel trying to be a good friend to John. Owen helps John deal with his immediate grief by taking the dress-makers dummy out of John's house because it might remind John too much of his mother. Owen helps John through school. Owen devotes quite a bit of his time through the years helping John to discover the truth of who is father is, and when they learn the truth, he helps John deal with that. Owen ultimately takes action that keeps John out of the Vietnam war.
Harriet Wheelwright is John's grandmother and she is the person who first really accepts Owen for who he is. Owen is an odd boy with a very mature mind and a very annoying voice, but Harriet doesn't pay any mind to that -- she has a good-natured grumpiness about the boys and their antics, but she seems to see something in Owen that is more than what appears on the surface. She provides a motherly figure for John as he grows up without his mother, and she provides a structure and stability to John's childhood.
Dan Needham is John's stepfather, and he does a wonderful job being there for John after his mother's death. He finds a wonderful balance between being a father-figure and being a friend. Because he and just gotten to know John when Tabitha died he doesn't try too hard to impose himself in John's life, but he provides for him throughout his life. He serves as a solid role model for both John and Owen.