Three point is the seat lightly in the saddle. A "modified" three point is just the crotch making light contact.
On XC, I almost never use three point. I am usually going at speed and I want no seat contact in the saddle. I may, on approaching a fence, dig a little seat in to help the leg, but only briefly. I want the horse's back to have as little interference as possible. Every time the horse has to push into a seat, it uses valuable energy on his part.
On stadium, I may use three point a little more, but not during the whole course. Two point is still my norm.
Dressage is the three point time! http://www.bing.com/videos/search?q=...iding+position