A federal jury in Newark found three powerful New Jersey Mafia members guilty in the slaying of a mob-connected businessman and an unfulfilled plot to kill a rival Mafia figure. Louis A. (Bobby) Manna--reputed No. 3 leader of the Genovese family--was convicted in the plot to kill John Gotti, reputed head of the rival Gambino family, and Gotti's brother, Gene. The jury also found Manna, 60, of Jersey City, guilty of plotting the 1987 murder of Irwin Schiff, of racketeering conspiracy, racketeering, taking construction payoffs and running a gambling business. Other mobsters convicted of the Gotti plot, Schiff murder and racketeering conspiracy were Hoboken restaurateur Martin (Motts) Casella, 72, and enforcer Richard DeSciscio, 42, of Bayonne. Former Hoboken police Lt. Frank (Dipsy) Daniello, 68, was convicted of the Gotti plot and racketeering conspiracy. Union official Rocco J. Napoli, 42, of Secaucus was convicted of racketeering conspiracy and accepting payoffs.