Why do tennis players get three balls and then chuck one away before serving??  

I don't watch a lot of tennis but I have seen a fair bit in my time and I must say I have never noticed that, do they all do that or just a couple that you have noticed, I hope someone in here has an answer for us...very interesting.

According to my knowledge they will check the hardness of the ball. it will be useful while serving. For the long serve may be they go for hard ball or vice versa. that may be the reason.

i think it's just for checking what the balls are like...if they like the look of them XD...or if they have the right ammount of skin/fur/the green stuff/ on the ball, because it can effect the spin of the ball and the bounce on it

As the other responses have indicated it is to check the balls inorder to find the best one before serving. They want to be sure the ball is good, sometimes a ball can be broken or may have gone flat. Also, they want one that is not ruffed up because it will go through the air faster and allow them to get a harder, faster serve. You may have noticed that most players are not so particular about the ball for the second serve when they care more about getting it in than about serving so hard and fast.