You could use the presentingViewController 'title' property to steer the unwind process to the desired originating ViewController. This solution would also allow you to pass data from your ViewController C to ViewControllers A and B.
1) Select the View Controller button (the yellow one) at the top of the storyboard canvas of ViewController A.
2) In the Attributes Inspector (View Controller section) give ViewController A a title.
3) Include the following code in ViewController A:
@IBAction func returned(segue: UIStoryboardSegue) {
// Don't need any code here
}
4) Repeat steps 1, 2 and 3 for ViewController B.
5) Override the prepareForSegue function in ViewController C as follows:
override func prepareForSegue(segue: UIStoryboardSegue, sender: AnyObject?) {
if self.presentingViewController!.title! == "ViewControllerATitle" {
let destination = segue.destinationViewController as! ViewControllerA
destination.someFunction(someArgument)
}
else if self.presentingViewController!.title! == "ViewControllerBTitle" {
let destination = segue.destinationViewController as! ViewControllerB
destination.someFunction(someArgument)
}
}