C172 INSTRUCTOR AND STUDENT HAD LESS THAN LEGAL SEPARATION WITH 3 ACFT IN HIO CLASS D AIRSPACE.

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I WAS INSTRUCTING AN INST STUDENT IN A C172 ACFT X; EXECUTING AN NDB APCH TO HILLSBORO; OR. ACFT X MADE THE INITIAL CALL TO HILLSBORO TWR. TWR RESPONDED WITH 'RPT 2 MI FINAL.' TIME WAS STARTED AT ABATE LOM AND ACFT X DSNDED FROM 2900 FT MSL TO 1020 FT MSL. APCH SPD WAS 90 KTS. TIME WAS COUNTING DOWN FROM 4 MINS. AT APPROX 1 MIN 30 SECONDS REMAINING; AN ACFT WAS SEEN CLOSING RAPIDLY AND IN A CLBOUT FROM RWY 30 AT HILLSBORO. IT EXECUTED A R TURN. ACFT X DOGLEGGED TO THE R AND CONTINUED THE APCH ON THE S SIDE OF RWY 12. SHORTLY AFTER SEEING THE DEPARTING ACFT TURN; A SECOND ACFT WAS SEEN CLBING OUT FROM RWY 30. IT; TOO; CLBED AND TURNED R. SHORTLY THEREAFTER; A THIRD ACFT WAS SEEN. IT FOLLOWED THE FIRST TWO. ACFT X CALLED MISSED APCH AND STARTED TO CLB AND TURN TOWARD THE NEWBERG (UBG) VOR. ACFT X WAS DIRECTED TO CALL PORTLAND APCH. SHORTLY AFTER CHKING IN WITH PORTLAND APCH; THEY REQUESTED THAT I CALL HILLSBORO TWR UPON LNDG. THAT WAS DONE. I SPOKE TO A GENTLEMAN. WE SPOKE ABOUT WHAT HAD TRANSPIRED. HE INDICATED THAT I SHOULD HAVE CALLED ABATE LOM. I REALIZED THAT THE FAF OR THE OM ON A NON PRECISION APCH SHOULD BE CALLED. I HAVE BEEN FLYING INTO HILLSBORO FOR SEVERAL YRS AS A FLT INSTRUCTOR. ANY OF SEVERAL POINTS ARE REQUESTED TO BE CALLED WHEN ON THE INBOUND LEG TO RWY 12. WHEN ABATE IS NOT CALLED; NO CAUTIONARY STATEMENTS HAVE EVER BEEN OFFERED BY THE TWR. OTHER POINTS THAT ARE OFTEN REQUESTED FOR CALLOUT ARE THE SUNSET HWY AND AS IN THIS FLT; 2 MI FINAL. AT OTHER TIMES; THE TWR DIRECTS THAT IT WILL CALL MISSED APCH. SINCE THERE IS NO DME FUNCTION FOR RWY 12 INTO HILLSBORO; THE ONLY WAY TO CALL A 2 MI FINAL IS TO ESTIMATE IT. I FOLLOWED THE LEAD OF THE TWR AND INTENDED TO ATTEMPT TO CALL A 2 MI FINAL. I FEEL THAT WE WERE OUTSIDE OF THE REQUIRED RPTING POINT WHEN THE INCIDENT OCCURRED. AT NO TIME WHILE ON TWR FREQ DID I HEAR ANY CONVERSATION BTWN THE TWR AND ANY OTHER ACFT.