Although I don't do AM, I have worked on more than a few local radio operators radios. Had a couple of requests to put echo boards into AM-only radios, as I refuse to do that to a SSB radio. Guess it is because I have too much respect for SSB operators.
The instruction for either the -2 or -3 are the same, IIRC. The real difference is in the chip they used. The -3 chip is the latest generation chip, and generates even less self-noise/distortion than the -2. Either one is much better than the Galaxy echo boards because of the chip quality. The RF Limited boards have a much cleaner sound - IMO.
Now, I am no expert in this matter to begin with. But if someone wants an echo board installed, I tell them to get the RF Limited board if they want to have a nice result.
As far as the features go, don't know if there is any difference between the -2 and the -3.