The threefold law was taken out of context from a book called High Magic Aid written by Gerald Gardner.
The popular misconception that there is a Wiccan Rule or Law of Three or Threefold Return comes from a misinterpretation of a passage in a work of fiction written by Gerald Gardner, the grandfather of modern Wicca. The book was called High Magic’s Aid, and he wrote it with the permission of his High Priestess. It had to be fiction because at that point, witchcraft was still illegal in Britain. In that book and its fictional story, the protagonist undergoes a sort of initiation rite in which he is taught “mark well when thou receivest good, so equally art bound to return good threefold.”
The witch is herself the agent of a threefold response, not the universe. Thus, when a witch receives anything positive; the witch is bound to return the favor threefold. However, anything negative sent out to someone, and that someone finds out; the witch deserves the retribution for sending out a poorly spell.
Wicca: Threefold Law
The positive side of Wicca is a nature-based 'religion'. Wicca is an offshoot of traditional british witchcraft wrapped in ceremony and pseudoreligion. I would recommend reading A Witches' Bible: The Complete Witches' Handbook by Janet Farrar and Stewart Farrar for further information on Wicca.
The negative side of Wicca is the neo-Wiccans who preach that Wicca is all "goodness and light" and often refuses to learn, refuses to think, and refuses to consider the possibility that they could possibly ever be wrong.
Charging is fine. There's nothing wrong with charging salt or crystals, in fact many charge crystals during a full moon to empower and infuse them with energy to work with specific properties. However, when one is using their own energy to enchant or empower you might need to cleanse yourself from any negative attachments both mentally and physically. That's how I feel anyway.
I've never heard of any negative experiences through prayer.