Last night I was talking with one of my team sponsors/"semi-elite" racer from the 1980's about the differences between lower "Cat" crits and upper "Cat" crits. We agreed that they are totally different animals not only because of the skill level, but also due to the length. In the lower "Cats" the pace usually starts out hard and gets easier. For the upper "Cats" it usually starts out easy and gets harder and harder.
At Cat 3 you are on your own.