SunTrust Banks of Atlanta said it has agreed to acquire Third National Corp. of Nashville, Tenn., in a stock swap valued at more than $750 million . . . Northrop and the French aircraft firm Dassault-Breguet are holding talks on possible cooperation on Dassault's new Rafale fighter, a Dassault spokesman said . . . Quotron Systems and AT&T ended a joint venture that had planned to develop and market information services products . . . Triton Group said its Continental Graphics unit will buy U.S. Press from Intermark Inc. for stock worth $31.2 million . . . The American Electronics Assn. reported that sales of U.S.-produced electronics products and services fell 3.4% during the first six months of 1986 from a year earlier . . . Alaska Air Group said it has purchased a total of 1 million shares, or 18%, of Jet America's common stock . . . Delta Air Lines said it has agreed in principle to sell its fleet of 13 DC-8-71 aircraft to United Parcel Service for undisclosed terms.