The nation's economy continued to expand in September, with strong gains in production, new orders and employment boding well for the fourth quarter, a trade group reported. The National Assn. of Purchasing Managers said its monthly indicator of future economic growth hit a three-year high of 60.7% last month, up from 59.9% in August. It was its highest reading since April, 1984, when it registered 61%. "The economy ended the third quarter with what must be characterized as robust growth," said Robert J. Bretz, chairman of the group's Business Survey Committee.