The judge in the Iran-Contra case said Thursday that the congressional immunity granted Oliver L. North and two co-defendants could make trying the three on conspiracy charges a "practical impossibility." U.S. District Judge Gerhard A. Gesell raised the problem at a hearing on the difficulty resulting from immunized testimony given to Congress by North, a former National Security Council staff member; former presidential National Security Adviser John M. Poindexter, and businessman Albert Hakim.