This is quite easy if you know the basics in Flash, and I am pretty sure there are dozens of flash tutorials on how to do this, code on Frame when assuming you have given the three buttons the instance names, my_btn1, my_btn2, my_btn3, and the textfield an Instance name of, my_txt:
Actionscript Code:
my_btn1.onPress = function(){
my_txt.text = "You pressed button 1";
}
my_btn2.onPress = function(){
my_txt.text = "You pressed button 2";
}
my_btn3.onPress = function(){
my_txt.text = "You pressed button 3";
}