OK, that was pretty easy actually.
For testing the functionality I made a plane with a 3D Text attached. The correct key to press is displayed in the 3D Text. If you press the correct key, the plane is translated -1 in Y.
First I set up my inputs in the input manager (Edit > Project Settings > Input) I just deleted everything and then set the size to 3 for my 3 keys required. I named these the letter and then set the input as the letter.
Then in my FSM I've got a Select Random String action with 3 strings set to each of my letters. This is then saved to a variable called ButtonKey_string.
Then I've got a Set Property action to set the text component of the 3d text to the value of ButtonKey_string. (Read this for how to use Set Property with 3d text -
http://hutonggames.com/playmakerforum/index.php?topic=2050)
Then I've got a Get Button Down looking for the button name ButtonKey_string which sends a transition to a new event that does my action. In this case, moves my plane -1 in the Y. And that's the basics covered.