Sales of manufactured goods rose 0.6 percent in December, the third consecutive monthly increase, adding a positive sign to recent mixed signals on future economic activity.

The Commerce Department reported that the December sales rise followed an increase of 1.0 percent in November. The government also reported that inventories rose a modest 0.3 percent in December, which followed a 0.2 percent increase in November.

In a separate report, the Federal Reserve Board said consumer credit rose $6.95 billion in December, an 18.1 percent annual rate. The December increase followed a $7.95 billion rise in November, a 21 percent annual rate.

Earlier this week, the government reported that retail sales rose 0.7 percent in January but that clothing and department store sales plummeted. Excluding automobile sales, retail sales declined 0.1 percent last month.

Economists said that the statistics this week suggested that retailers, facing disappointing sales in December and January, are trying to work down their stocks and may continue cost cutting that began after Christmas to get rid of their unwanted inventories.

Inventories of wholesalers and manufacturers are doing well, said Robert Ortner, the Commerce Department's chief economist.

The speed at which retailers can clear their stock depends on consumer spending, which, while not strong, should continue modestly this year, economists said.

Manufacturers' and trade inventories were estimated at $566.5 billion, little changed from November but 10.1 percent higher than in December 1983, the Commerce Department said. Retailers' inventories rose 1.6 percent, and those for wholesalers declined 0.7 percent.

Manufacturers' inventories were virtually the same as in November, Commerce said. Durable goods inventories rose 0.7 percent in December, and nondurables showed little change from November. The increase in sales of manufacturers' goods in December followed declines in July, August and September and rises in October and November.

Durable goods sales were little changed from November. Wholesalers showed the largest change, with a 0.8 percent decline in sales, Commerce said. Retailers' sales dropped 0.7 percent in December and those of manufacturers were virtually unchanged.

Nondurable goods sales rose 1.3 percent from November, and those of wholesalers rose 2.8 percent. Manufacturers' nondurable sales increased 1.7 percent. "Retailers showed only a moderate change," Commerce said.

Meanwhile, the Fed reported that automobile credit outstanding rose $2.07 billion in December following a $2.72 billion rise in November.