The site will announce changes but not all changes are mentioned, similar to the original lyrics of the theme song "and all the rest on Gilligan's Isle".
See here http://blog.chess.com/webmaster/live-chess-gets-some-love
Does not address the specific question of three-move resignations but if you track webmaster there may be fewer surprises.
It seems that chess.com recently changed the 3 moves and resign with no penalty rule. I often play chess on a computer with a poor touchpad. During the opening when I'm trying to play out mindless moves without wasting time I very occasionally slip. Just earlier I had white and played 1 e4 e5, and meant to play 2 nf3 but accidentally played 2 f3. It's obviously a totally worthless move, so I resigned and immediately asked for a rematch. I was stunned to see that I had lost rating points.
Now, if the rule is that you lose points no matter when you resign, then fine, but there really needed to have been some sort of notification that that was going to be the new rule. I'm not complaining because I'm obsessed about rating. I just don't want to feel pressured to play out a game in which I made a hugely influential mouse slip on the second move. But my main point is not complaining about the new rule - it's fine - I just think there ought to be a notification for something like that.