It's possible that the three sides represented three seasons: wet, dry, cold (or a variant on the theme). It may also have represented the three trimesters of human pregnancy. Given these two possibilities, it might have been an early form of a calendar, in my opinion.
In modern times the triskele symbolizes a group of three. The user may select whatever trio is special to them. Christians might select God, Jesus and the Holy Ghost. Wiccans might select the three stages of the Goddess (Maiden, Mother, Crone), which some refer to as the Triple Goddess.