"Union casualties have been grossly exaggerated and probably did not exceed 3,500. Commentators have suggested numbers ranging from 7,500 to well above 12,000, all supposedly incurred during a few terrible minutes after dawn. (In reality the assault sputtered on for about an hour, not the eight minutes some writers have claimed.) The 12,000 estimate in fact reflects the official tally of all casualties for the entire Cold Harbor campaign--12,788, to be exact--embracing the cavalry battles of May 31 and June 1, the 6th and 18th Corps attacks at Cold Harbor on June 1, the 5th and 9th Corps fights around Bethesda Church from June 1 forward, and casualties in all Union corps for the remainder of their stay at Cold Harbor and during their movement across the Chickahominy and James Rivers twoard Petersburg. The oft-quoted 7,500 figure derives from attempts by early postwar writers to estimate all Union losses for June 3, a day that included a great deal more fighting than just the grand assault." [Gordon C. Rhea, Cold Harbor: Grant and Lee, May 26-June 3, 1864, p. 359.]
Rhea later writes, "All things considered, the grand charge at Cold Harbor on June 3 produced about 3,500 Union casualties." [Ibid., p. 362]
Mike Gorman and Gordon Rhea are talking about the grand assault with respect to the 3,500 casualties.