switching power supply breaks under too small load?
Most probably some of the output electrolytic capacitors have failed due to aging. This results in large voltage ripple, which messes up the feedback and average voltage  measured with a multimeter drops to 3V, while maximum voltage still would be about 5V if you look at it with an oscilloscope.

You have a big chance to repair these chargers successfully just by replacing the old caps with new. Be sure to use high temp (105C) and LOW ESR type capacitors. Be aware that there is High Voltage on PCB. After unplugging the adapter from mains you should wait long enough for it's input capacitor(s) to discharge.