You can stack two but it will require modifying to board. They are designed to be modified for pin change, but you still would have to run some wires. To use all 8 pins for each driver, you will only have two pins left on the Arduino, which would probably be 0 and 1, for programming/serial.
Stacking three would be difficult. There are only 18 digital outputs (including the analog pins), which is enough for PWM and direction control of three motors, but no enable pins or current sense. It uses every pin, which means you will have to disconnect at least one board to program the Arduino.
You would probably be much better off using the drivers' back connector and wiring all three boards to a Mega, which has enough pins to fully control all six motors.