MSI Data said net earnings for the third quarter of fiscal 1988 rose 425% to $1.47 million from $280,000 for that period in the previous year. The Costa Mesa-based manufacturer of portable data terminals said sales for the quarter ended Dec. 31 increased 32% to $23.87 million from $18.08 million for that period in fiscal 1987. For the nine months, earnings rose 405% to $3.43 million, and sales increased 34% to $66.42 million. MSI said its gross profit margins increased in the quarter and over the whole nine months. The gains were attributed to higher sales and a cost-reduction program.