Despite being overpaid though, would most people not agree that all 3 have positive trade values? Its not like you have to add to have someone take Marner's contract, right?
There are wildly different opinions of Marner's value. What makes a good trade is trading someone for what they think he's worth, plus a little more if you can get it, rather than what you think they're worth. That also provides you some insurance in case you're the one who is wrong.
As for Marner specifically, I would say it's almost universally accepted he is overpaid, even his biggest fans have trouble justifying his contract, but I've heard from multiple people that his real money owed after his bonus this July will be $6 per year. A lot of teams that value real money, the cheap ones like Arizona, would love that bang for the buck. Still, even if I have extra cap space, I don't give that away for free when I can sell it instead.