Like other publications of the Academy, such as the Diccionario de la lengua española de la Real Academia Española, the work follows a linguistically prescriptive philosophy as opposed to a descriptive one.
The first one is the most straightforward, the second is a variant plural formation found commonly in words ending with a stressed -í, whereas the third is the most unusual and the least recommended (Real Academia Española's Diccionario panhispánico de dudas labels it "vulgar in appearance").