Distributed Logic Corp. said it has completed its $7.5-million acquisition of three subsidiaries of the Plessey Co. of Britain. Distributed Logic is an Anaheim-based manufacturer of computer disk, tape and communication controllers. The three Plessey subsidiaries, with annual sales of $50 million and "break-even" profits, are located in Irvine and in Ontario, Canada, and the United Kingdom. . . . Resdel Industries, a Newport Beach supplier of communication equipment, is negotiating to sell a newly created issue of $10 million of convertible preferred stock for a combination of $5.5 million in cash and 443,100 shares of its common stock. Reliance Financial Services Corp. said in a filing with the Securities and Exchange Commission that it is considering the purchase of Resdel's new stock issue.