How can I get all three Kalos starters?
There are a few different ways to obtain all three Kalos starts.
Your rival will trade you the starter that is weak to your starter after beating the Elite Four. -NOTE: You need a spare Pokemon to trade for it, but you should have one.
Friend Safari can give you the 1st evolution of a starter if you are lucky with who you add as a friend.
Wonder Trading can result in one of the Pokemon you are looking for, but it requires lots of luck, and lots of attempts.
Using the GTS is an easy way. If you breed your start and put it to trade for one of the ones you are looking for, it could be traded.
If you have a friend who has a different starter who would be willing breed it and trade one, then trading is another way.
Now out of those, the GTS is the best way to get the starters, seeing that there are many breeders and collectors who are willing to trade their starters for other Pokemon.
It really takes lots of patience, and to look at the right places.
Just like the previous Pokemon games, you have to trade with others to obtain the other two starters. I've actually gotten very lucky with the new Wonder Trade option on the PSS, so you could try that out if trading isn't proving lucky.
Not only trade!
You can trade and earn another starter - in my case Fennekin - after you beat the game (win with Elite Four) from one of your friends when you go out from home. This might be connected with beeing nice to your friends all game and choosing "nice" dialogue options.
I'm not sure if you can earn third starter after beating Elite Four second time... but it will be great :)