May I suggest that a stellium is composed of
Four or more planets in one sign (or house...not as strong) of which at least two are not Sun, Mercury or Venus. Sun, Mercury and Venus are constant companions and frequently inhabit the same sign, which is why a true stellium demands that at least two of the planets be other than these three.
Three planets do not constitute a stellium, but a very strong multiple conjunction. But this conjunction does not (normally) exercise the overwhelming influence of a true stellium. If there are three planets in one sign and a fourth in the opposing sign it is of very similar effect as the stellium, with the emphasis and power given to the single planet and its sign. The same stricture on which planets participate applies.
Planetary formations represent psychological complexes (I do not use this term with the negative implications often associated with it). A stellium is a planetary formation just like the t-square, grand cross, grand trine, yod and the others. In most formations there is a quality of simultaneity: the involved planets are triggered at more or less the same time by transits, and possibly directions, depending on the orbs present in the formation. Activation of the first element of the formation, provided it is connected by effective natal orbs, will stimulate the entire complex and sets off a chain of action-reaction-further reaction within the personality and environment in a pattern of circularity. The parts of the formation-complex act as one, that is, directly affect one another through the resonance set up by their proximity (by degree) and intimate relationship. If one of the planets tends to predominate in the formation, the effects will probably be most noticeable (strongest) when this planet receives direct stimulation from a transit or direction because it "commands" the formation.