Hi LJGMDAD,
If they were both on a solid surface then a single 3" core would be firmer than a 6" core because it would be compressed to a greater percentage of it’s total thickness (see post #14 here about the effect of thickness).
If you have two 3" layers that are both the same ILD as a single 6" layer they would be very similar. There is more about two 3" layers vs a single 6" layer in post #2 here.
Phoenix