Too complex you don't need arrays just do:-
{
xValue = yValue = zValue = 0;
for (byte i=0; i<AVERAGE_READINGS; i++){
xValue += analogRead(Xpin);
yValue += analogRead(Ypin);
zValues += analogRead(Zpin);
}
xValue = xValue /AVERAGE_READINGS;
yValue = yValue /AVERAGE_READINGS;
zValues = zValue /AVERAGE_READINGS;
As the analogue read is only 10 bits you will not overflow the value of the int until AVERAGE_READINGS is greater than 32. If you want more than this just make the readings a long int.