This basic circuit can be adjusted for almost any power level: low power - Help please! I need to give constant current to an LED from a Lipo battery - Electrical Engineering Stack Exchange
You need to size the MOSFET, heatsink and shunt resistor appropriately.
Shunt resistor of 5 milliohm and 3W would give 100mV for 20A load... Be sure to use and understand 4-terminal
resistors...