how to increase the possibility of getting a specific random object from an array

Generally, I know that I can get a random element of an array in the following way:

var myArray = [x, y, z]
let myArrayLength = UInt32(myArray.count)
let myArrayIndex = Int(arc4random_uniform(myArrayLength))
let randomElement = myArray[myArrayIndex]

But how can I make the possibility of y being the random element twice the possibility of x and z being the random element thrice the possibility of y?