I take it you are wondering if all the parties involved are cheated..

In the context of cheating where no permission to step outside the relationship has been given then yes all parties involved are being cheated.

The partner being cheated on is having their trust betrayed, the partner doing the cheating is just screwing themselves over by having to lie and the person who is the other is being cheated by either being lied to (being told that their partner is single etc) or being used strictly to satisfy a need or whim (although not always)