Agree, 8.65 acres if there is zero overlap and maintain constant 3.4 mph.

Calculate 3.4 mph x 3hrs = 10.2 miles traveled

10.2 miles x 5280 ft/mile = 53,856 ft traveled

53,856 ft x 7' cutting width = 376,992 sqft mowed

376,992 sqft divide by 43,560 = 8.65 acres mowed