Help regarding the Hitman series

I was thinking of playing Hitman Series, but I am confused now, I read somewhere that they combined the 3 parts into 1 and now it's called World of Assasins. I was under the impression that Hitman 1,2,3 are seperate?

Should I just get the World of Assasins? Or is there some other chronological order i should play the game in ?