Ford Motor Co.'s third-quarter net income fell 17.5 percent, down to $313 million from $379.7 million reported in the year-ago period, company officials said yesterday.

The latest quarter's earnings translate to $1.70 a share of common stock, compared with $2.05 a share in the third quarter of 1984.

A cut-rate financing program implemented to match an aggressive marketing campaign launched by General Motors Corp., the nation's largest auto maker, cut deeply into Ford's earnings. But the company's performance also was hampered by a two-week September strike that shut down its assembly operations in Lorain, Ohio.

Ford produces its Thunderbird and Mercury Cougar passenger cars and its Econoline Vans and Club Wagons -- some of the company's most profitable models -- at the Lorain plant.

New tooling and other costs related to 1986-model production also ate into profits, according to Donald E. Petersen, Ford chairman and chief executive officer.

Ford officials said in interviews with The Washington Post last week that they had not wanted to follow GM's 7.7-percent auto financing campaign.

GM officials said they started the marketing effort to clear out a backlog of 1985 cars and trucks left as the result of a lengthy Teamster's union auto hauler's strike last summer. But Ford officials argued privately that such a campaign wasn't needed.

The official said that Ford "was forced" to match GM because Ford's dealers were worried that the low GM-financing rates would lure too many customers.

But GM, too, paid dearly for the incentive campaigns. GM on Tuesday reported third-quarter profits of $516.5 million. But that money came largely from tax breaks, improved foreign exchange rates and other sources. GM had third-quarter operating losses of $20.9 million in its business -- auto manufacturing.

Ford's sales for the quarter totaled $11.6 billion, down 2 percent from $11.8 billion in the year-ago period. The company reported $155.2 million in operating income, less than half the $362.7 million in operating income received in the year-ago quarter.