You know, if you had to pick only 3 exercises that you could do, what would they be?
1. Pull ups
2. Push ups
3. Dead lifts (even though I hate them and never do that)
I think with those 3, you could probably hit most everything.
If you're only doing 3?
1. Push ups
2. Sit ups
3. Cardio of some type
Core strength is probably the most important aspect to being in shape. Your core helps you bend, twist, breathe, sit up, stand up straight, and on and on and on.
Not to hijak the thread more than I have, but situps would actually rank pretty far down on the list; there are so many better/more efficient exercises to work your core that will also benefit multiple other areas of your body (deadlifts, overhead squats,etc.).
Also, have to be a tad more specific than just cardio. To be honest, when I train for running races, I may only run two days a week, including race day. Getting a good kettlebell regime going not only greatly benefits cardio but will tone the hell out of a lot of your body, especially in areas that are oftentimes neglected.