I find that if I do all of the core exercises in one day, the final exercise suffers as a result. It seems more intuitive to me to break things up. I am doing one day with a core and an upper exercise, a second day with the second core and upper exercise along with the lower, and the third day with the third core and upper exercises.
Does that make sense? Or do you recommend doing all the core at once and all the upper at once?