LM317 construct 3volt output and 2amp output
The answers are focusing on your questiong about configuring the LM317 correctly. However, I'd rather go for your intent.

Just because the stepper motor nameplate says it runs at "2.4V @ 2A" does not mean that is exactly required. It really means that at a supply voltage of 2.4V, it will draw 2A.

You can run the motor at a higher voltage (IME it's rare to run at nameplate voltage) and this will provide better high speed performance, however you must limit the current to avoid motor overheating. So if you run your "2.4V" motor at 12 volts, either a power resistor must be used inline with the motor coil, or a current limit driver must be used instead of your L293. Using a resistor is often called the "L/R" method.

Let's say we wanted to keep your motor running on 4.8 watts (2.4 * 2) from a 12V power supply. We'd want to reduce the current to 0.4A. That would mean a 48 ohm resistor (50ohm is close standard value) with a 2W rating. Motor will likely run much smoother.

It is worth investigating more modern drive IC's than the L293 that incorporate a current-limiting chopper drive. But as a start, the L/R method is fine.