I think Tunney was an immense boxing talent. His levels of concentration, combined with his spatial awareness, made for some really clever footwork and positioning. That said, both Frazier and Foreman were machines and I think Tunney's fleetness and guile could only take him so far, making these fights a little interesting. More so for Foreman than Frazier, I suspect. Tunney can't hurt either of them and this would be to his detriment against two attacking boxers like Frazier and Foreman. I would actually suggest that Frazier's swarming would be devastating to Tunney. Frazier just runs Tunney over (then reverses back over him and then maybe forward again, to finish the job). Foreman would have his patience tested; in part, because his approach would provide more room for Tunney to maneuver, defend and negotiate some kind of offensive response to the Foreman juggernaut. But, sooner or later, the mid-round engagements lead to Foreman clipping Tunney, which starts the ball rolling in a downward direction, towards a stoppage defeat.
Tunney's strategy is irrelevant. He hasn't got the power to hurt them, therefore he can't beat them. Foreman mauls him in the first round. If Tunney runs, perhaps he lasts 2 or 3. And Frazier gives him a proper beating, more so than Foreman, who doesn't get the chance to do so, since he takes Tunney out with the first big punch he lands.