Your requirement doesn't make any sense to me unless i am missing something here.
You have a combo chart, assuming Year as a dimension and couple of metrics, 1.sum?? 2. Average.
So sum is a bar? and plots a bar for every year and an average as a line and you want to plot 3 year average. This is where i am getting confused. so if you do average you will one value for each year. Or if you manage to aggregate then you will get one value. not sure how you are ogign to trend this.
Do you have any other dimension beside Year in you chart? Perhaps a sample app would be good to see.