A B757 FLC IS CONFUSED REGARDING TA'S AND DURING A DISTR WITH WAKE TURB ENCOUNTER THEY EXPERIENCE AN NMAC WITH A COMMUTER FLT WHILE ON APCH TO RWY 25L; E OF LAX; CA.

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WE WERE ON APCH TO LAX RWY 25L. AT ABOUT 25-30 MI OUT; WE WERE GIVEN A TFC CALL FOR A COMMUTER FLT 11 O'CLOCK POS AND AT 10 MI. WE SEARCHED BUT DID NOT SEE THE TFC. THE COMMUTER FLT CALLED 'TFC IN SIGHT.' WHAT I NOW REALIZE MUST HAVE HAPPENED; THE COMMUTER ACFT SAW THE B757 WHO WAS AT 3 1/2 MI IN FRONT OF US. ABOUT 1 MIN AFTER THIS EXCHANGE; OUR B757 ENCOUNTERED THE WAKE TURB OF THE B757 IN FRONT OF US. OUR AUTOPLT DISENGAGED AND I PROCEEDED TO HAND FLY THE APCH. I ELEVATED 1/2 DOT ABOVE GLIDE PATH TO GET OUT OF WAKE TURB. SINCE I WAS HAND-FLYING; MY OUTSIDE SCAN WAS DEGRADED. A MIN OR TWO LATER WE GOT A TCASII WARNING. THE DISPLAY SHOWED AN ACFT SLIGHTLY AHEAD TO THE L AT 500 FT SEPARATION. WE SEARCHED BUT DID NOT SEE THE TFC. WE CALLED TWR AND ASKED IF THERE WAS TFC IN OUR VICINITY. HE RESPONDED 'YES THE COMMUTER.' AT THIS POINT; COMMUTER WAS DIRECTLY ABOVE US WITH TCASII SHOWING 200-300 FT SEPARATION. TCASII CALLED OUT; 'DSND; DSND;' SO WE DSNDED. WE THEN STARTED OPENING FROM TFC. CAPT THOUGHT THE COMMUTER CAPT WAS SHOWING OFF FOR A NEW FO. I NOW REALIZE; HE NEVER SAW US TO BEGIN WITH. SOLUTION: 1 ACFT CALLING TFC IN SIGHT IN A SIT SUCH AS THIS SHOULD NOT RELIEVE ATC OF THEIR TFC SEPARATION RESPONSIBILITIES. ALSO; IF A PLT CALLS TFC IN SIGHT IN THE APCH ENVIRONMENT HE IS WAY TOO BUSY TO STARE AT THAT TFC INDEFINITELY.