Why is Buddhism, Jainism, Hinduism so related?

When I thought of it it is all "ahems" (peace).

Buddhism and Jainism were founded by men who were raised Hindu but felt that there was something more in life that they were missing. Neither one set out to create a new religion or even philosophy, they were looking for their own answers, just like any one of us. Hinduism wasn't just their religion, it was their culture too, so it was hard even if they wanted to completely shed their old way of life, so remnants from Hinduism stayed with them and their new way of life.

Ahimsa is peace, but more literally it's "non-violence"

Because Buddhism and Jainism (as well as Sikhism) are "subsets" of Hinduism. They retained most of the spiritual philosophy but broke away from the ritual aspects. Buddha was a practicing Hindu as was Mahavira, the alleged founder of Jainism.

All three faiths come from the same place, India. Both Jainism and Buddhism were outgrowths of Hinduism, the same way that Christianity and Islam were derived from Judaism.

I don't know what you mean by "Jainism" but I know about Hinduism and Buddism. They both don't really believe in God but believe something else.

Geographical, they all originated in India.
Jainism is Prue Aryan, Hinduism is descended from the Indo European religion brought by the Aryans mixed with native Dravidian elements and Buddhism is the philosophical off shoot form Northern India.