Access aggregation formula

I have been asked to produce a table of values which has me stumped. The table has daily quantities for multiple meters and they want to see three-day quantities for groups of meters. Meters are assigned a group code so group-by on each day is easy. Any way I can get sum of a group of meters for three days, record that sum, roll forward one day and record the sum of that three days, etc for each meter-group?