The Immune Wars - a European conflict first alluded to in the second season - resulted in a huge power shift in Europe, which is why the US and UK have no real resources or equipment there - there are still holdouts from the group that was the 2nd season villain - and organized governments are pretty much non-existent outside of a few that rebuilt after the virus breakout and the Immune Wars. The number of American military personnel who survived the virus numbers in the thousands and though they should have plenty of ships and military vehicles to use, their manpower levels for the entire armed forces at the end of season 3 probably don't even add up to a single base, if you don't include the rebuilt Army/Marines/whatever they call themselves. This is why they are using any able-bodied military veterans who retired and survived, like the pilot of the Huey and the Huey itself, as the pilot wouldn't be certified to fly a Seahawk and those are probably in use at the few US Naval bases up and running. If you listen carefully in the 3rd season US scenes, you hear that the continental US is largely desolate after the virus and the few regional centers of governance and industry are mostly along the coast, with some interior cities beginning to be rebuilt and repopulated based on their location/ability to manufacture or process resources for use by the new government. As those cities come online again, you'll see more military resources being absorbed from left-over National Guard and US Army posts, as well as whatever Air Force bases are not up and running. The real issue is availability of military-grade fuel and munitions, as there would only be a finite supply of both, particularly fuel, as no refineries have been mentioned as being functional at this time.