According to Politico, the Commission on Presidential Debates announced on Friday 9/16 that the same requirements will be used to evaluate candidate eligibility for the 2nd and 3rd Presidential Debates:
"[T]he Board determined that the polling averages called for in the third criterion are as follows: Hillary Clinton (43%), Donald Trump (40.4%), Gary Johnson (8.4%) and Jill Stein (3.2%). Accordingly, Hillary Clinton and her running mate, Tim Kaine, and Donald Trump and his running mate, Mike Pence, qualify to participate in the September 26 presidential debate and the October 4 vice-presidential debate, respectively. No other candidates satisfied the criteria for inclusion in the September 26 and October 4 debates. The criteria will be reapplied to all candidates in advance of the second and third presidential debates."
As far as for when they will make the determination, I imagine that it would be similar to the first. Approximately 2 weeks before each debate, the commission will look at their "five selected national public opinion polling organizations" to determine the polling eligibility requirements of the 4 candidates. Those who make the cut will be invited and the rest will not. Going over to the commission's website doesn't give exact dates.
Invitations to participate in the second and third of the CPD's scheduled presidential debates will be based upon satisfaction of the same multiple criteria prior to each debate.