In season 1, he appeared in most episodes, but as he became busier (he is a Broadway actor, after all), he was unable to appear with the same frequency.
Since there is more than one Observer, and that Cerveris can't always be there for small cameos, it is not only convenient, but makes sense mythology-wise that there would be multiple Observers observing multiple events.
Hence the different actors (and the digitally-inserted appearances from time to time ).
I'm sure that as Season 3 progresses, that we'll not only see September and those we have seen so far, but be introduced to new ones.
Hope this helps.
Also, I have no knowledge of a Season 3 episode being titled "September". There was an episode in season 2 titled "August", which focused on the Observer of the same name, but I haven't heard any news of a September-centric. However, that does not mean that it will happen (in fact, I readily welcome it ).