３つ以上のオブジェクトの衝突

タイトル通りでございますが
３つのオブジェクトの衝突について
当方はかなり手こずりましたので、投稿させていただきます。

import SpriteKit

class GameScene: SKScene, SKPhysicsContactDelegate {

let whiteCategory:  UInt32 = 0x1 << 0
let blackCategory:    UInt32 = 0x1 << 1
let blueCategory:   UInt32 = 0x1 << 2


override func didMoveToView(view: SKView) {
    

    self.physicsWorld.contactDelegate = self
    
    self.size = view.bounds.size
    self.physicsBody = SKPhysicsBody(edgeLoopFromRect: self.frame)
    self.physicsWorld.gravity = CGVectorMake(0.0, -3.0)
    

    let blackSquare = SKSpriteNode(color: UIColor.blackColor(), size: CGSizeMake(50, 50))
    blackSquare.position = CGPoint(
        x: CGRectGetMidX(self.frame),
        y: CGRectGetMidY(self.frame)
    )
    blackSquare.physicsBody = SKPhysicsBody(rectangleOfSize: CGSizeMake(50, 50))
    blackSquare.physicsBody?.affectedByGravity = false
    blackSquare.physicsBody?.dynamic = false
    

    blackSquare.physicsBody?.categoryBitMask = blackCategory
    blackSquare.physicsBody?.contactTestBitMask = whiteCategory
    

    let blueSquare = SKSpriteNode(color: UIColor.blueColor(), size: CGSizeMake(50, 50))
    blueSquare.position = CGPoint(
        x: CGRectGetMidX(self.frame) - 100,
        y: CGRectGetMidY(self.frame) - 100
    )
    blueSquare.physicsBody = SKPhysicsBody(rectangleOfSize: CGSizeMake(50, 50))
    blueSquare.physicsBody?.affectedByGravity = false
    blueSquare.physicsBody?.dynamic = false

//(2)
// blackSquare.physicsBody?.categoryBitMask = blueCategory
// blackSquare.physicsBody?.contactTestBitMask = whiteCategory

    blueSquare.physicsBody?.categoryBitMask = blueCategory
    blueSquare.physicsBody?.contactTestBitMask = whiteCategory
    

    self.addChild(blackSquare)
    self.addChild(blueSquare)
}

override func touchesBegan(touches: NSSet, withEvent event: UIEvent) {
    

    for touch in touches {
        
        let location = touch.locationInNode(self)
        let whiteRectangle = SKSpriteNode(color: UIColor.whiteColor(), size: CGSizeMake(50, 50))
        whiteRectangle.position = location
        whiteRectangle.physicsBody = SKPhysicsBody(rectangleOfSize: CGSizeMake(50, 50))
        

        whiteRectangle.physicsBody?.categoryBitMask = whiteCategory

// whiteRectangle.physicsBody?.contactTestBitMask = blackCategory
// whiteRectangle.physicsBody?.contactTestBitMask = blueCategory

        self.addChild(whiteRectangle)
    }
    
}

override func update(currentTime: CFTimeInterval) {
}


func didBeginContact(contact: SKPhysicsContact!) {
    
    var firstBody, secondBody: SKPhysicsBody
    

    if contact.bodyA.categoryBitMask < contact.bodyB.categoryBitMask {
        firstBody = contact.bodyA
        secondBody = contact.bodyB
        //thirdBody = contact.bodyB
    } else {
        firstBody = contact.bodyB
        secondBody = contact.bodyA
       // thirdBody = contact.bodyA
    }
    

    if firstBody.categoryBitMask & whiteCategory != 0 &&
        secondBody.categoryBitMask & blackCategory != 0 {
            //secondBody.node?.removeFromParent()
            println("black")
    }
    

    if firstBody.categoryBitMask & whiteCategory != 0 &&
        secondBody.categoryBitMask & blueCategory != 0 {
            //secondBody.node?.removeFromParent()
            println("blue")
    }
}

}