A320 FO BELIEVES QUESTIONABLE SEPARATION OF ACFT ON CONVERGING VISUAL PATHS TO ONT WAS THE RESULT OF POOR ATC BY APCH AND POOR SITUATIONAL AWARENESS ON THE PART OF OTHER FLT CREWS.

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WE WERE ON THE ZIGGY4 ARR INTO ONT. SHORTLY BEFORE DAWNA; WE WERE CLEARED BY SOCAL APCH TO DSND TO 11000 FT. APCH CALLED OUT TFC; A B737; AT OUR 2 O'CLOCK AT 13000 FT. THE TFC WAS SOME DISTANCE OFF AND WE DID NOT SEE IT AT FIRST. CONTINUING INBOUND; WE SIGHTED THE ARPT AND THEN THE B737. THE CAPT CALLED BOTH THE ARPT AND TFC IN SIGHT AND WE WERE CLRED THE VISUAL APCH RWY 26R INTO ONT. WE WERE INSTRUCTED TO MAINTAIN VISUAL ON THE B737 AS WE WERE #2 FOR THE APCH. THE B737 WAS ALSO CLRED FOR THE VISUAL APCH. APCHING FROM OUR 8 O'CLOCK WAS A B747 FROM THE SE WHO WAS ALSO CLRED THE VISUAL (#3; BEHIND US). WE CONTINUED THE ROUTING OF THE ZIGGY4 INBOUND TOWARDS PETIS AND THE SEQUENCED TRANSITION FOR THE RWY 26R APCH. WE STARTED SLOWING TO 220 THEN 180 KIAS ALMOST IMMEDIATELY AND SET THE 4200 FT PETIS CROSSING ALTITUDE INTO THE ALT WINDOW. WHILE SEVERAL MILES FROM PETIS; I OBSERVED THAT THE B737 WAS NOT PROCEEDING INBOUND TOWARDS THE FINAL APCH FIX (FONTA) BUT WAS PROCEEDING OUTBOUND TOWARDS PETIS AND THAT WE WERE ON A CONVERGING COURSE AND CO-ALTITUDE. WHEN WE WERE CLRED FOR THE VISUAL; WE EXPECTED THE B737 TO FLY TO THE FAF (FONTA); NOT TURN OUTBOUND TOWARDS PETIS CREATING A CONFLICT WITH OUR ACFT. IF HIS ARR AIRSPEED AND 2000 FOOT HIGHER ALT CREATED A TIGHT SPOT FOR HIM TO GET DOWN WITHOUT TURNING INTO OUR FLT PATH; HE SHOULD NOT HAVE ACCEPTED THE VISUAL CLRNC. NOTING THAT OUR FLT PATH AND THE B737'S PATH WERE CONVERGING; I STARTED A TURN OFF THE ARR PATH AND TO THE SOUTH (LEFT.) THIS CREATED AN ISSUE AS THE B747 WAS NOW AT OUR 9 O'CLOCK AND CONTINUING INBOUND ON WHAT WAS; FOR THEM; MOSTLY A STRAIGHT IN VISUAL APCH. THE B737 ACFT CAME NO CLOSER THAN 1-2 MILES; BUT HE HAD NO SITUATIONAL AWARENESS OF WHERE WE WERE OR THAT HE WAS TURNING INTO OUR FLT PATH. APCH CONTROL NEVER QUERIED THE B737 AS TO HIS FLT PATH OUT TOWARDS PETIS. APCH; AFTER WE TURNED APPROX 45 DEGS TOWARDS THE S; AUTHORIZED US TO FLY S OF COURSE. THE B737; IN TURNING OUTBOUND TOWARDS US; SO REDUCED THE SEPARATION BETWEEN US; THAT THERE WAS ONLY 2 MILES SEPARATION FROM US EVEN AFTER WE OFFSET S OF COURSE TO MAINTAIN SEPARATION. THUS; WE WERE RECLEARED FOR THE VISUAL TO LAND ON RWY 26L. APCH ALSO HAD TO VECTOR THE B747 OFF COURSE TO MAINTAIN SEPARATION. A SUBSEQUENT; NON-EVENTFUL VISUAL APCH AND LNDG WAS MADE ON RWY 26L. SEVERAL THINGS CONCERN ME ABOUT THIS SITUATION. 1.) SOCAL APCH: IN RELATION TO ONT ARPT; THERE WAS A THE B737 DUE N AT 13000 FT; AN A320 AT 11000 FT NORTHEAST ON THE ZIGGY4 AND A B747 SOUTHEAST; ALL GETTING CLRED FOR VISUALS. APCH IS THE ONLY PERSON WITH A TRUE PICTURE OF THE RELATIONSHIP OF CONVERGING ROUTES AND CLOSURE RATES AND HAS NO RESPONSIBILITY AS HE HAS CLRED EVERYONE FOR VISUALS. (A FAR DIFFERENT SCENARIO THAN THE NORMAL VISUAL CLRNC AT MOST ARPTS) WHEN ACFT ARE COMING IN ON A SIMILAR FLOW PATTERN. TO HAVE 3 CONVERGING ACFT IS A LARGE SAFETY ISSUE. SOCAL APCH SHOULD HAVE BEEN MONITORING THE 3 ACFT AND CALLING THE B737 WHEN HE WENT TOWARDS PETIS; 5.3 MILES OUTSIDE THE FAF (FONTA.) 2.) THE B737; EARLY ON; HE WAS ADVISED WE WERE THERE BUT HAD NO SITUATIONAL AWARENESS THAT HE WAS TURNING OUTBOUND INTO OUR ARR PATH. 3.) OUR ACFT; WE HAVE ACFT CONVERGING FROM BOTH OUR R AND L. WHEN THE B737 TURNS INTO OUR FLT PATH; THAT LEAVES US PINCHED WITH NO PLACE TO GO WITHOUT CREATING ANOTHER CONFLICT.