Who told you that ?
Arduino Due: https://store.arduino.cc/products/arduino-due
It has already a step-down converter. A MPM3610 makes 5V from VIN. A normal linear voltage regulator NX1117 makes 3.3V from the 5V.
That means you can use up to 12V and it will not get hot. The 5V and 3.3V output can deliver 800mA. In my opinion, the 5V can deliver 800mA, but the 3.3V with 800mA might be too much for the voltage regulator.
Is the backlight of the TFT screen using a lot of power ?