I would be fine with something like this, with a slight tweak, for the general election.
For the first debate, the requirement would be as they say. Ballot access in states totalling 270 electoral college votes. If more than one candidate meets that requirement, the one with the most signatures becomes the 3rd wheel in the debate.
For the second debate, an additional requirement would be needed. The individual would need to be polling at least at 5% in 3 nationally recognized polls (create a list of applicable polls for this purpose).
For the third and any subsequent debates, an additional requirement would be needed. The individual would need ot be polling at least at 10% in 3 nationally recognized poll.
Here's my logic...
First, I agree with them that they're looking at just 3. More than that and it muddies the water of the debate far too much and does an ultimate disservice to the voters.
Second, the first debate should have a low threshold to get into. This is essentially a third party candidates "coming out" party. Their first big chance to hit a national audience. To expect them to be polling even mildly well prior to this point is likely unreasonable.
Third, after the first debate...IF the candidate is even MILDLY viable...he should begin to poll at least a noticable level given the additional exposure from the first debate. If by the second debate the candidate still can't manage 5% support in the polls, or by the third debate he can't manage 10% support, then to me that's a clear indication that the candidate has little realistic chance to win the Presidency and is not catching hold with the American Public. As such, I believe it's in the Publics best interest for a candidate that so few are interested in, and has such a implausible chance of winning, to be taking up valuable air time that could be spent allowing the Public to better learn about the two primary candidates.