How to make USB smartphone charger?
The cell phone charger spec is a maximum. I.e. the maximum current that can safely be drawn from the charger is 2A. If the buck converter is rated for 3 amps, then you have more than enough current available to charge your phone. Your phone will draw the current it needs.

Some phones (especially Apple devices) are finicky about the data lines. You can't just leave them floating (with nothing connected). Adafruit has a good discussion about reverse engineering the voltages that are required on the USB data lines for charging Apple devices.