actually historically, magick was used by the oppressed of society(yes i know the elite practiced it too)
there was no concept of the law of three (because wicca hadnt been invented yet)
and the whole personal gain thing also sprung about the same time as the law of three, its a new age belief, but not an intrinsical magickal law.
the belief that if you do magick to gain for yourself it will backfire is a very new belief, and one that is not historically supported, that also goes for the law of three, generally only wicca, and some new age sub-groups believe in the instant karma of the law of three, besides that i know hinduism and a few other eastern religions believe that what you do causes karma, but not in this life, instead it affects in how "advantageous" you will be born in the next life
so to answer your question, the law of three (instant karma) is a wiccan concept, so if your not wiccan, it doesnt apply.
the personal gain thing is a new age concept popularized on the t.v. show "Charmed" and has no historical basis, almost all ancient magick was used for personal gain.