Re: flying with a 3mo old?
I'm really surprised by the number of people who had to remove babies from a carrier (I'm not talking an infant bucket seat, but an Ergo). I've done it 4 times without ever a question or hesitation. The TSA website says:
"Infants and small children may be carried through the metal detector, but if the alarm sounds, the officer will have to conduct additional screening on both the passenger and the child. If a baby is carried through the metal detector in a sling, additional screening may be required even if there isn’t an alarm"
The additional screening was always swabbing my hands for explosive residue (takes 2 seconds). It's worth a shot - not sure if you want to print out the TSA guidelines to bring, but it sure is a heck of a lot easier to leave them attached! No one even batted an eye, though, when I did it.
You cannot, however, wear your baby during take off and landing in any type of sling or carrier. I usually leave my carrier on, though, so when she falls asleep I can pull the straps up and have my hands free (just not during take off and landing).
I would consider buying a cosco scenara, leaving it packaged, and checking it at check in rather than renting.