According to
USHoleInOne.com, which insures hole-in-one contests, the odds of getting a hole in one on a par-3 are 12,500 to 1 for an amateur and 7,500 to 1 for a pro - how they work that out I really don't know.
Here's a good article on the topic on usatoday.com's site:
http://www.usatoday.com/sports/golf/2006-07-16-hole-in-one_x.htmI think it's a mixture of the two mind you the best players get more than average players - so more skill than luck for me.