Hi Saiva,
I think that it is definitely either a "lucky mutation" as Kat put it, or a separate plant in the pot as Ross pointed out sometimes happens.
I actually had that happen to me. I bought a phal in bloom, that produced what I thought was a basal keiki. When the basal keiki bloomed, it was soooo different that I knew it had to be a separate plant, only it grew much slower. When i repotted it after it finished blooming, I indeed had 2 separate plants rather than a single plant with 2 crowns.
I guess one way to rule out one way or the other would be to find out if the 3 keikis were stem props or basal keikis. If they were all stem props...meaning the keiki formed off of an old flower stem, then it is a "lucky mutation", otherwise it is probably a separate plant growing alongside the larger mother plant.