Another debate can only benefit Trump. Biden already well ahead.
The reason is that another Trump performance anywhere close to the last one will re-extend Biden's lead, increasing the chances of getting the 52+ US senators needed for a true majority (because of Manchin and Sinema, who voted with Trump half the time). It increases the chance of getting a win on the night of November 3 or during November 4th.
Even if he agrees and Trump either refuses because of rule changes or fails to get the test prior to the debate, as required, it is a win, if not as good as another Trumpish performance.
The risk of Trump's suddenly turning on a rational being and coping with his mic's being turned off when he wants to interrupt is pretty small, though non-zero.