Fall Race Schedule?

I am looking for a little advice:

I am scheduled to do an Oly in June and a half Iron in July.  I really like the Oly distance and this will be my first attempt at a half.

This fall there are 3 races I am looking at:
Pumpkinman (Maine) 1/2 Iron Sept 12th 
the Lobsterman (Maine) Oly Sep 19th
NH Marathon (New Hampshire) Oct 3rd

I can't do all three and would like to try a Marathon this year.  I think I could do the Oly Sept 19th and Marathon Oct 3rd.  Do you think I could do the 1/2 iron and then the Marathon or is that to much?  Or would I be better served just concentrating on the Marathon for the fall?