Has a 20% chance to burn, freeze, or paralyze the target. One of these effects is selected at random; they do not each have independent chances to occur.
from veekun. judging by this description, the game first makes a roll on 20% to see if an effect is inflicted, then makes a second roll out of 3 to see which effect is chosen. however to be certain you'd need to examine the game code itself... things like these can be finicky.