CBS Inc. reported a third-quarter net loss of $114.1 million today, the largest quarterly loss in its history.

The loss reflects the company's decision to get out of the toy, computer software and theatrical film production businesses, as it moved to pay for its successful defense against a hostile takeover attempt.

After CBS defeated Atlanta broadcaster Ted Turner's takeover bid last summer by adding hundreds of millions of dollars of debt to its books, the company said it planned to sell $300 million of assets to reduce the debt.

In a related development, Loews Corp. Chairman Laurence A. Tisch was elected to the CBS board of directors today. Tisch, who owns 11.9 percent of CBS and plans to increase his stake in the company to 25 percent, is expected to bring stability to a company that has been the subject of intense takeover speculation for almost a year.

There also has been some press speculation recently that Tisch ultimately may seek control of the company and replace Thomas H. Wyman as chairman. Wyman has been the target of criticism since CBS began firing employes as part of its program to reduce operating expenses by $20 million.

When asked about Tisch joining the board, Wyman said at a meeting today with Wall Street analysts, "He [Tisch] was very well behaved this morning. I think he enjoyed his first meeting. He sees his role to perform as an outside director."

Wyman said that the divestitures announced today will enable CBS to focus on its three core businesses; broadcasting, publishing and recorded music.

He said that CBS will receive about $100 million in cash and tax benefits from the sale of its toy business. However, he also said that the ailing business should have been sold a year ago, which would have reduced CBS losses by $25 million.

CBS recorded a $114.6 million loss in the third quarter from discontinuing its toy operations. The company said it has agreed to sell certain assets to Hasbro Inc., while negotiations are continuing for the sale of other toy business assets. Wyman said today the company had to decide in 1982 whether to get out of the toy business or expand. "We took the wrong fork in the road and purchased Ideal [a toy company] in 1982," he said.

CBS recorded a $21.1 million loss in the third quarter from discontinuing its theatrical film operations. This includes losses from discontinuing the company's theatrical films unit and the anticipated gain from the sale of its stake in Tri-Star Pictures, a joint venture with Time Inc. and Columbia Pictures. The company's theatrical films unit has produced 12 movies since 1979, none of which has been profitable at the box office in the United States.

CBS recorded a $7.5 million third-quarter loss from discontinuing its software operations. The unit has developed and marketed entertainment and educational computer software for the home since 1982. Wyman said the company's execution in this venture was excellent, adding that it failed because the overall market failed to materialize.

"We will continue to take risks of this kind," Wyman said, referring to CBS Software. The company said it will continue to sell and service existing software.

CBS financial results also suffered because of weak network advertising revenue, delays in the release of albums, and expenses related to the Ziff-Davis magazine acquisition earlier this year.

Overall, CBS had revenue of $1.12 billion and a net loss of $114.1 million ($4.55 a share) in the third quarter, versus revenue of $1.08 billion and net income of $48.8 million ($1.64) in the same period last year.

For the first nine months of 1985, CBS reported revenue of $3.41 billion and a net loss of $28.1 million ($1.06) versus revenue of $3.31 billion and net income of $176.3 million ($5.93) in the same period last year.

Its income from continuing operations was $147.2 million for the first nine months of 1985, compared with $187.9 million for the same period a year ago.

Wyman said today that the company expects to earn between $1.75 a share and $2 a share in the fourth quarter, which means earnings for the year are expected to be less than $1 a share. In 1984, CBS earned $7.15 a share.

CBS stock responded to the announcements today by rising $2 to $120 a share. On Tuesday, the day before the announcements, the stock dropped $2.25.

CBS chief financial officer Fred Meyer said today the company had $4 million in expenses in the third quarter directly related to fighting Turner's takeover bid. He said "total defense spending" to thwart hostile bidders in 1985 will be about $12 million.

Before responding to questions from analysts today, Wyman reflected on the difficult year that 1985 has been for CBS executives.

"But," he added, "with our business portfolio realigned as per today's announcements and the finanical consequences thereof behind us, we're really looking forward to 1986. It has to be a lot better than 1985."