Here's another twist on this, from Gmommy7702:

We are having a problem understanding a play, first a 3 was played and then a 2 and then a 1 then the next person played a 2 and was told that they earned another 3 points.

could you please let me know what the rules say about this play?

I find it helpful to think about it the way it's phrased above:

Each card played scores points for any run it completes

In other words, if it doesn't complete a run of 3 or more cards, it doesn't score. The sequence 3-2-1 is a run, so the player playing the 1 pegged 3 points. The player who played the 2 after that did not complete a run (the 3-2-1 run had already been completed and scored) so should not have pegged any points.

Actually, this answer is slightly incorrect. Player 2 does NOT get a point for go at the time that Player 1 announces the go. Player 2 must play out the remaining playable cards (in this case, both of his remaining cards) before taking a point for the go. He does get the 3 peg points for the run of 3 (A-2-3) and 4 more points for the run of 4 (A-2-3-4) and THEN he gets 1 point for last card. He earns a total of 8 points in pegging this hand.