OPERROR NMAC INVOLVING ACR X ON VISUAL APCH TO PARALLEL RWY OF ACR Y AND ACFT Z ON VISUAL APCH BEHIND ACR Y.

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COPLT WAS FLYING. WE EXPERIENCED SEVERAL DELAY VECTORS AND WERE KEPT HIGH; APPARENTLY DUE TO TFC. FROM AN E DOWNWIND AT 5000 FT TO RWY'S 16; WE WERE TURNED BASE HDG 250 DEGS ASSIGNED SPD WAS 210 KTS. APCH ASKED IF WE HAD ARPT AND MD-80 AT 1 O'CLOCK. WE REPLIED THAT WE HAD THE ARPT AND WERE LOOKING FOR THE TFC. WE LOCATED THE MD-80 TFC WHICH WAS APPARENTLY ON EXTENDED FINAL FOR RWY 16R; BUT WE WERE UNABLE TO RPT IT DUE TO RADIO CONGESTION. WE WERE GIVEN CLRNC FOR THE VISUAL TO RWY 16L BASED ON THE MD-80'S SIGHTING OF US. I CONSIDERED THAT OUR TFC WAS CLOSE AT 1 O'CLOCK LOW; THAT A SHARP 90 DEGS DSNDING L TURN WOULD PUT OUR BELLY TOWARD THE MD-80; AND THAT WE MIGHT WELL OVERSHOOT HIS FLT PATH. MY REPLY TO APCH WAS THAT WE HAD THE TFC IN SIGHT; ACKNOWLEDGED THE VISUAL TO RWY 16L; BUT WOULD CONTINUE OUR HDG LONG ENOUGH TO PASS ABOVE AND BEHIND THE MD-80. WHEN MD-80 PASSED FROM R TO L; WE BEGAN OUR TURN FROM BASE TO FINAL. OUR FLT PATH CROSSED BOTH FINALS IN THIS TURN AND PUT US CONSIDERABLY W OF THE RWY 16R FINAL DURING THE TURN. WHILE TURNING; WE NOTED THAT A THIRD ACFT WAS CLRED FOR A VISUAL AFTER RPTING 'THE B757 AND MD80 IN SIGHT.' THIS CLRNC WAS TO RWY 16R BEHIND MD80. SINCE WE WOULD BE XING THIS FINAL; I ATTEMPTED TO LOCATE THIS ACFT BEHIND US TO THE L; BUT WAS UNSUCCESSFUL. AS WE ALIGNED WITH RWY 16L AND SLOWED AT BOEING; WE GOT A TCASII ALERT. I NOTED SEVERAL TARGETS; ONE OF WHICH WAS COLLOCATED WITH OUR ACFT SYMBOL READING PLUS 3. IMMEDIATELY A CONFLICT WARNING 'DSND NOW' WAS ANNOUNCED. COPLT MOVED FORWARD WITH THE YOKE AND DSCNT RATE WAS INCREASED. TCASII SEPARATION DECREASED TO PLUS 1. COPLT SAW A B737 ACFT PASSING OVERHEAD. HIS HDG WAS SLIGHTLY GREATER THAN OURS; AND HIS SPD SOMEWHAT HIGHER. COPLT NOTED HIS GEAR EXTENDING. THE B737 LANDED ON RWY 16R. WE WERE BTWN A HALF A MI AND A MI OUT WHEN HE LANDED. WE LANDED ON RWY 16L. THERE WERE NO COMS WITH US FROM ATC ABOUT THIS ACFT.