Before the attack you make a lot of noise about how you will now spend the rest of the game (and the next few games if you are really annoyed) taking this person out, blah blah blah. Then during the attack its best to keep quiet and let the thing play out, seizing on any opportunity for counter threat offered by someone else (like if you go too far player 3 will duff you up in Australia etc), you can hmm and nod along (without looking too desperate).
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After the attack, (if you survive) a depressed reaction with comments like "oh well, I'll get you next time" etc, again a natural reaction, also underline the fact that this act of aggression has a long term impact for the aggressor. However your immediate priority is to survive, so you'd better get used to going quiet and contenting yourself with other peoples conversations.
This is where Asia is such a great place to hide out in all but the end game. It's usually a mush of various players with no real intent on taking the continent for themselves (yet), and it's now your job to regroup there and encourage in fighting elsewhere on the board.
As I said earlier, the first few turns after the defeat need to be spent quietly and wisely. You are now looking to take any fragments of your empire from various places around the board to this Provence. You must expect most of them to be killed off.
Therefore your minimum 3 reinforcements must always go on the same Provence in Asia (or if you are lucky to have two close by, split them once the first provence looks strong relative to those around it). This idea can work very well in a 3 player game.
I remember being mashed by a good player who was in South America and I was in Africa, he pretty much did me on the first move, so I got out of the way and regrouped in Asia. I played on the fact that this player was now in control of 2 continents and shouldn't player 2 do something about this?
The two of them were very concerned about not letting the other one get too powerful, so little wars here and there broke out. Meanwhile I was able to collect my 3 each turn and not lose out too much in the overall troop count. In the end I made a bid of Australia and got away with it on a turn after a bigger battle between the two. I then concentrated on gaining Asia.
In the end I took Asia and after much too-ing and fro-ing managed to hold it, over take the other two and win the game.
It is possible to come back from the dead at Risk, but it's not a position I would ever choose to be in.