You haven’t been doing it wrong. As per my understanding, you aren’t doing it “wrong” unless you don’t include the head, general area of the sternum, or the shoulders (in the order of head, sternum, and shoulders left to right). Even then, it would only be “wrong” if you messed it up during a time that the Church specifies that the Sign of the Cross is done, like Mass, confession, etc. Anything added on to the sign is up to the individual and is optional.
(It’s different for Eastern Catholics, but I chose to omit this because it will confuse the issue, as the OP is very likely Latin Rite, and possibly doesn’t know of the traditions of the eastern Church.)
That being said, I do the three/two finger thing, as it brings to mind the three persons of the Trinity, and two natures of Christ.