Keep in mind that many of the programs have activity requirements to keep your miles from expiring. USAir requires at least one earning or redemption activity every 18 months. I don't know about American and United; Amex points expire if the account lapses.

It's not that complicated to generate a single transaction. Most of the airline programs have tie-ins to restaurants. Find a friend in the USA who has three credit cards, and register one with United, one with USAir, and one with American. Once every 17 months have them go to one of the restaurants, go up to the bar, and buy three sodas, one with each credit card. (Or ask the bartender for three glasses of water and a bill for each one, and tip him a couple bucks on each one.) Poof, you have your "earnings activity." Points.com is also a good one, for example, you can "spend" 5 USAir points to get one Air Canada point and thus get a "redemption" activity that way.