Well...it technically STARTS at 3pm, but like the PP said, depends upon where you are viewing the parade...it may 'start' at 3:05, or 3:10, or ever 3:15...keeps things interesting!
That reminded me of a funny story. The receptionist in the place that I worked was religiously following the O.J. trial. (the first one) The day that the verdict was supposed to be announced, she couldn't wait for the announcement. She asked for the millionth time what time the announcement was going to be made. We responded that it was going to be at 3 pm (our time), 12 pm California time.
Her first reaction was..."that's not fair, how come they get to know 3 hours before we do". Kind of speaks for itself, don't you think?
If you include the time you go reserve a spot and wait for the 3pm parade in your calculations, then it begins at 2pm (or even earlier in some instances).
What's funny about this is that the cast members have to be trained to give a certain response. Like it starts at 3 but will go past that spot at 3:15.