E135 FLC INITIATE EVASIVE MANEUVER DUE TO CONFLICTING B757 WHILE ON APCH TO DFW RWY 17C.

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WE WERE CLRED FOR A VISUAL APCH TO RWY 17C. I WAS EXTRA ALERT; BECAUSE I JUST HEARD A SIT OVER THE FREQ WHERE AN SF340 (ACFT Z) WAS VECTORED TOO LATE ONTO THE LOC AND AN ACR HAD TO CLB DUE TO AN RA ON HIS TCASII. ATC AND ACR WERE ARGUING ON THE FREQ. SUDDENLY; I SAW AN ACR B757 (ACFT Y); A COUPLE MI IN FRONT OF US; DIVING OUT OF 3500-3800 FT ON A VISUAL APCH TO RWY 17C ALSO. WE WERE OVER(UNDER) TAKING THAT TFC FAST. I CALLED ATC AND RPTED TFC. I ASKED WHERE THE TFC WAS GOING; NO ANSWER; JUST A TURN TO 130 DEG HDG (FROM 175 DEG HDG); AND THEN BACK TO 240 DEG HDG TO RE-INTERCEPT. (I KNEW THEN THAT THE B757 WAS ALSO GOING TO RWY 17C...AND WE WERE ABOUT TO BE PANCAKED BY A B757.) IT WASN'T GOING TO WORK. WE WERE TOO CLOSE BEHIND THAT B757 AND STILL OVER(UNDER) TAKING! (I WAS REDUCING SPD FAST.) ATC REALIZED IT AND GAVE US 080 DEG HDG; GAR; UP TO 3000 FT. AFTER LNDG; I CALLED ATC ON THE PHONE; TO RPT INCIDENT AND FIND OUT WHAT HAPPENED. ON AUG/MON/01; I LEARNED FROM OUR ACR; THAT THE CTLR IS RECEIVING EXTRA TRAINING AND INTERNAL AUDIT IS UNDERWAY.