I assume you are asking about serving as the president of the board of these organizations and not as a full time CEO president.  The latter would be very hard to do, although I recall the story told to me by a university counsel about the accountant who was “full time” on the faculty at the counsel’s institution, “full time” on the faculty of a nearby college, and conducting a “full time” accounting practice, all at the same time.  Even in higher education that is difficult to do, and when it was discovered, it cost the accountant at least one of his full time faculty positions.

I am not aware of any statute or regulation that would prevent you from serving as a board chair type of president in more than one 501(c)(3) or other nonprofit organization.  Unless you are retired or otherwise unemployed, however, I would think twice about doing it.  Being a board chair is a serious position and usually demands a substantial commitment of time.  You don’t want to be thought of like that accountant.