I have a portable power bank and I can charge devices via USB on it. Is there an adapter to change a 3 prong charger to a USB, so I can plug my Mac into my power bank? A Mac air charger that can plug into a USB port would work as well.
Most MacBook use MagSafe charging which doesn't operate on the same voltage as USB - so the vast majority of battery banks that charge anything USB won't work. There are some inverters that would power the adapter using 120 or 240 VAC but those are quite large.
Only the new MacBook with USB-C connector is low power enough to charge over the new power delivery specification for USB 3.1 that generally is supported along with the change to USB-C connector. Any charging bank would need increased power than what is used to charge most any mobile devices made in 2015 and earlier.
Standard USB ports can not handle the load of power that a Macbook Air/Pro draws. So no, it is not advisable even if you could find an adapter. (Note that if you have the new ultrathin Macbook—not the Air or Pro—you can use a USB-C to USB2 cable to charge it from a power bank.)
You can get a special power bank (like this one) that can charge your Macbook Air via Magsafe.