Multiple time frame indicator based on Average H/L range

A simple indicator in my eyes but what I am looking for is an indicator to display the average range of the previous H/L for the last x amount of bars, then divided by 2. The an example formula for this would be ((bar1+bar2+bar3/3)/2).

I need the indicator fixed on the open of the new bar, displaying the range above and below the open of the bar. For example, if the average range for 3 bars were 40 pips or points, a line or whatever would display 20 pip/points above and below of the open bar(which would have a line showing the the open also). Ill attach an example.