Cover 1: Congress had the franking ability for surface mail. If they desired a letter to go via airmail, then they had to pay the postage, as in this case. It looks like the airmail stamp partially covers the facsimile signature.
Cover 2: National Surety Co - a normal airmail cover of the time.
Cover 3: First flight cover for a new airmail route. The cover is rough, but finding the enclosures is a very nice bonus.
-edited post now that I understand the corrected title.