A USB port on a computer is typically limited to 500mA unless the device is intelligent, talks to the computer, and requests more. Standalone USB wall supplies or even powered hubs can support more, you will need to look on the specific device to see what it supports. It is not uncommon for a USB wall supply to support 1-2A. As for the 3A requirement, that is based on what you are using at the output
Input Voltage * Input Current * Efficiency = Output Voltage * Output Current
For example lets say you want 11V at 1 A and your converter is 85% efficient
5V * xA * .85= 11V * 1A
Input current xA = 2.58A
Switching regulators are power converters, so power in = power out, minus some for not perfect efficiency.
