From the audience perspective, three is the magic number. Go for more and people get confused. Tell them to watch out for three things and they are primed to be attentive. They can visualize when three points come together. Revealing you have three items to discuss also builds a sense of anticipation for what is to come, especially if you leave your best point for last.
Interestingly, researchers have found most complex technologies and ideas can be described in three different statements or ideas most of the time.