Despite lackluster consumer spending and a disastrous stock market, National Lumber & Supply Inc., a chain of lumber and hardware stores based in Fountain Valley, posted its third consecutive quarter of increased sales and earnings. National Lumber posted a $61,397 profit in its fiscal 1988 third quarter ended Oct. 31, down 73% from the $228,476 posted for the same period in the previous year. However, the higher fiscal 1987 profits are attributed to gains from the sale of National Lumber property in Ontario and the closing of a San Bernardino store, company officials said. "We've cut our costs drastically by instituting a safety program that cut workers compensation and liability insurance premiums," said V.J. Carnevale, National Lumber's vice president of finance. "Our margins have improved. If we can keep our sales going, we'll be in fine shape." The company reported revenues in the period of $39.4 million, an increase of 3% over the $38.3 million reported in the same period a year earlier. Profits for the nine-month period were $458,190, up from a net loss of $456,798 for the same period a year earlier. Revenues for the period were $120.9 million, an increase of 9% over the $111.1 million reported in the previous year.