I'm with the previous poster, and would recommend that you take a day to visit each of them. If you try to combine them you'll miss too much! And they all involve a lot of walking, so you'd be really tired, too - though you'll still be pretty tired doing one per day.
If you're short on time you might want to consider doing either the Zoo or the Wild Animal Park but not both. If that's the case I would recommend the Zoo over the WAP, just because there's a wider variety of animals there. And it's a little smaller, too.