Hi,
I went to make a simple current limiter 3A and can deal with 12V.
What about using 2 parallel LM317T or is there are any better way to do this ?

 

How accurate does it have to be? Do you want a source or a sink?

If you don't need extreme accuracy BJT transistors are natural born current limiters. The parts count can be pretty low using them, even at high currents.

 

I will use it to supply a Unipolar stepper motor drive .
I will use 3 driver and each drive will put limiter to it.
I using tip122 to drive the motor and here the circuit

 

This may do what you need. Q2 in the circuit is your tip122 transistors. For a 3A limit, R2 should be ≈ 0.22Ω. R1 should be selected to give about 12mA base current into Q2.

 

I ran into problems trying to gang LM317's as current limiters (running them in parallel). It didn't work and seem to fry some of the LM317's.

 

This may do what you need. Q2 in the circuit is your tip122 transistors. For a 3A limit, R2 should be ≈ 0.22Ω. R1 should be selected to give about 12mA base current into Q2.

So what the value of R1, and which transistor Q1 that i should use ?
#Note that i will put the current limiter at the supply point

 

What action should the limiter do?

Go into constant current mode and force the current to not exceed 3A?

If you are working from the high side, you could used an unbalanced current mirror so that it ratios something like 100:1 and use the smaller current side to flag the current limit action.