SWIFT - Sprite Kit : Array with nodes at each index

i'm creating a game with Sprite Kit in Swift, and i've three nodes (jewels) generated at 3 x position :

self.size.width * 1/6  // left
self.size.width * 1/6  // middle
self.size.width * 1/6  // right

I've created an array with the three positons to call them randomly :

    let randomX = [self.size.width * 1/6, self.size.width * 3/6, self.size.width * 5/6]   // Array with three positions
    let randomIndex = Int(arc4random_uniform(UInt32(randomX.count)))
    let xPos : CGFloat = randomX[randomIndex]

I've an other array with the three nodes of jewels :

    let randomJewels = [SKSpriteNode(imageNamed: "jewel1"), SKSpriteNode(imageNamed: "jewel2"), SKSpriteNode(imageNamed: "jewel3"),]

And I set the position and the size of my generated node :

    let jewel1 = randomJewels[Int(arc4random_uniform(3))]
    jewel1.anchorPoint = CGPointMake(0.5, 0)
    jewel1.position = CGPointMake(xPos, self.size.height / 2)
    jewel1.size.width = 40
    jewel1.size.height = 20
    self.addChild(jewel1)

The three positions need to be taken by three random node (jewel) but if I generate two other jewelry, they may have the same position, as they may have three different positions. How can I do so that the jewels hold the three positions ? It can not be void position, there is a jewel on every x position !

Thanks !