Honestly don't worry too much if you can only pick up one of the three right now. The fact that they are still on shelves over a year and a half after their release is enough to tell us they exist in pretty high quantities and I am sure in the future you ll manage to track the ponies you dont buy off the shelves down through other collectors or ebay.
I would say pick the one which you think you will appreciate the most - dont worry about the show, dont worry about which one is the coolest - just pick the one which makes you smile and which you will miss not having in your collection.