Three officials of Lexicon Corp. have been appointed to the seven-member board of directors of Scope Inc., a Reston-based company that specializes in the application of computer and signal-processing technology.

Lexicon President Michael Levy is the new chairman of the board of Scope. Executive Vice President Timothy J. Crowe and an outside Lexicon director will join the Scope board by July 9 to give Lexicon majority control.

Scope had been the target of a hostile takeover by Lexicon, a Fort Lauderdale, Fla., producer of microcomputer-based data communications products, since last month when Lexicon began a $5 tender offer for Scope stock. Through a modification of that offer, Lexicon had increased its share in Scope from 27 percent to 44 percent by June 12. Lexicon now holds approximately 50 percent of Scope.

According to Scope, Lexicon has agreed not to take over the company before Oct. 31, 1986 without offering all Scope shareholders at least $5 cash per share, unless a merger is voted in by a majority of the non-Lexicon stockholders. Lexicon has also agreed to drop its demand for an Aug. 14 stockholders meeting to vote on a new board of directors.

Although Lexicon had stressed the compatibility of the technology of the two companies, Scope voiced a strong desire to remain independent. Scope President Edward C. Driese admitted that Lexicon had acquired enough stock to vote their candidates onto the board of directors.

"We believe, therefore, it is in the best interest of Scope, its shareholders and employes to work with the Lexicon organization to pursue our mutual business interest and we look forward to our relationship with Lexicon," Driese said.

"It is my intention during the next several weeks to work closely with the Scope organization to formulate plans for the future that will achieve our mutual business objectives," Levy said.

Scope stock closed Friday at $4.88 a share and Lexicon closed at $2.81 a share, both unchanged from the previous day.