How to find the yellow card
There's a building labeled "Private Property." Near it is a fountain which has an underwater passageway into the building. Inside the building are a bunch of C-9 canisters which you can blow up to reveal a switch. That switch will open up a hole containing the yellow key. You will have to leave the building and go to one of the courtyards to find it.
How to find the red card
The yellow card unlocks an underwater passage in the pool. Grab some steroids and follow the passage into the huge gray sparkly building. You'll need the steroids to get through before you run out of air. Inside the gray building, there's an elevator to the left of the water. It looks like an empty area, but you can take it up to where the red card is. You can't take the elevator down, so jump down and land in the water.
How to find the blue card
This one isn't hard; you can find it if you look around.
Okay, maybe you're not looking in the right place. When you insert the red card, go past the mountain to the ground beyond it. There's a button to push. It lifts the blue card to the top of the mountain.