Micro General said it lost $297,093 in the third quarter ended Sept. 27, contrasted with year-earlier net earnings of $4,317. Sales fell 22% to $870,497. For the first nine months of its 1987 fiscal year, the company reported a loss of $307,568, contrasted with net earnings of $10,551 in the corresponding period in 1986. The Irvine manufacturer of computer-based postal shipping and counting scales said revenues for the nine months were $3.1 million, a decline of 13% from year-earlier revenues of $3.6 million. Company officials attributed the decline in third-quarter revenues to "a shift in the sales mix of the company's parcel shipping systems towards the lower-end, lower-priced models." Sales of postal scales also declined, the company said.