step down 6 V 3AMP to 5 V 3AMP
The basic assumption of a resistor divider is that the current flowing through both resistors is the same. When the current is not the same, the resistor divider equation does not work.

6V is pretty close to 5V. You almost don't need a regulator. Instead of a normal resistive divider, make a divider with a diode "on top" and a 1k resistor "on the bottom." An ordinary silicon diode. A nice big one that can dissipate lots of heat without a separate heatsink. Look for something rated at 10A or something.







simulate this circuit &ndash; Schematic created using CircuitLab