To do that id recommend making the skript pick a random number and going off that... i know its ineffecient but its the best idea i have.
command /luck:
trigger:
set {_number} to %random integer between 1 and 100%
if {_number} is 1-20:
#do stuff
stop
if {_number} is 21-60:
#do other stuff
stop
else:
#do even more stuff
stop