The Law of Threefold Return
The Law of Threefold Return is not some mathematical formula. If you give someone $100, you do automatically get back $300. If you steal a $1.50 candy bar, you do not automatically have $4.50 stolen from you. Whatever energy you put does come back to you.
The idea behind this principle is simple, however. Newton observed that for every action, there is an equal and opposite reaction. In short, everything has consequences, including your words, thoughts, and deeds. If you cause harm to someone, you will have to pay for that action either in this lifetime or another (karma). If you are a Mother Theresa, this may just ease a future lifetime of yours in the sense of having already learned a particular set of lessons in this lifetime. This is action and just passively waiting or living your life; you are the actor in your life and you determine what energies you put--and receive--from the Universe. It is a form of reciprocity.