A BE1900 OVERSHOOTS THE TURN TO FINAL FOR RWY 3R AT DTW WHEN THE CREW WAS ATTEMPTING SOME UNNEEDED SPACING BEHIND A DC9 ON A VISUAL FOR RWY 3L.

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DTW APCH CLRED US FOR THE VISUAL RWY 3R; TO FOLLOW HVY DC10 TURNING R BASE. TO ALLOW SEPARATION FOR WAKE TURB; I PLANNED TO SQUARE MY TURN TO FINAL. AT THAT TIME DTW CLRED A DC9 FOR A VISUAL TO RWY 3R FOLLOWING HVY DC10. WE SPOTTED TFC (DC9) AT OUR 1 O'CLOCK POS INSIDE OF US FOR THE FINAL. MY INTENTION WAS TO CONTINUE ON BASE LEG; CLR OF THAT TFC UNTIL WE COULD CLARIFY SEQUENCE WITH ATC. WHILE WAITING FOR A CHANCE TO CONTACT APCH; WE PROCEEDED THROUGH THE FINAL TO ALLOW SEPARATION FROM THE DC9. APCH CONTACTED US BEFORE WE HAD A CHANCE TO CALL; AND THEY INFORMED US THAT THE DC9 WAS ON FINAL FOR RWY 3L; AND WE SHOULD BEGIN OUR TURN FOR THE RWY 3R FINAL. MY FO AND I WERE BOTH UNDER THE IMPRESSION THAT THE DC9 WAS ON THE FINAL FOR RWY 3R. WE BELIEVE A BREAKDOWN IN COM OCCURRED BTWN OURSELVES AND APCH. POSSIBLE CONTRIBUTING FACTORS: HVY RADIO TFC ON APCH FREQ. THAT MAY HAVE PREVENTED ATC FROM ADVISING US OF CONVERGING TFC. THE FREQ CONGESTION WOULD HAVE BEEN LESS IF HAD THERE BEEN SEPARATE FREQS FOR EACH RWY. IT WOULD ALSO MAKE THE DC9'S ASSIGNED RWY MORE EVIDENT.