The book-to-bill ratio, a key indicator of the health of the giant semiconductor industry, rose for the third straight month in March, the Semiconductor Industry Assn. reported. The ratio, which measures the number of new orders to shipments, rose to an estimated 0.86 for March, up from 0.79 in February and 0.65 in January. Those levels are still far below the 1.53 recorded in January, 1984, but the upward trend may indicate that the semiconductor industry is moving out of a yearlong recession. The book-to-bill ratio represents the percentage of new orders to shipments each month.