You have to declare your variables on the class. The way you do it now, you declare new variables in the methods and they are in scope only in those methods.
public class SlotMachine_V2 extends GraphicsProgram {
String aa;
String bb;
String cc;
...
}
Then you use them:
public void Wheel_1(){
int a = rgen.nextInt(1, 6);
if( a == 1){//BAR//
aa = " BAR, ";
...
}