MD88 AND 2 OTHER ACR ACFT HAD A TRIPLE COORDINATED TCASII RA IN ATL CLASS B AIRSPACE.

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ACR FLT ABC; APCHING ATL FROM SSW. VECTORED TO INTERCEPT RWY 9R LOC AT 3500 FT MSL. I WAS LEVEL AT 3500 FT. I WAS LEVEL AT 3500 FT IN AN APPROX 20 DEG R BANK ON DOGLEG. TCASII RA POPUP WITH THREAT ACFT 100 FT ABOVE AND BEHIND US. CAPT VISUALLY ACQUIRED ANOTHER M88; BELLY-UP TO US INTERCEPTING RWY 8 LOC AND ASSUMED THIS WAS THREAT ACFT. WE DID NOT INITIALLY RESPOND TO RA FOR THIS REASON. SECONDS LATER; AN ADDITIONAL RA TO 'DSND.' WE (AND THE OTHER MD88) WERE NOW LEVEL AND ALIGNED SO WE REALIZED THEY WERE NOT THE THREAT ACFT. I INITIATED DSCNT TO COMPLY WITH RA. CAPT NOTIFIED TWR WE WERE RESPONDING TO RA. I BELIEVE AN ACR ACFT BEHIND US RPTED CLBING DUE TO A TCASII RA. APPARENTLY (TWR LATER TOLD US) A VFR ACFT WAS TRANSITING THE TCA RPTED AT 3000 FT MSL (BELOW TCA). I THINK HE WAS ABOVE THAT ALT; AS OUR RA WAS TO DSND. COULD HAVE BEEN A XPONDER ALT RPTING ERROR THOUGH. IMPORTANT POINTS: TWR/APCH SHOULD HAVE WARNED OF THE VFR TFC. WE SHOULD HAVE RESPONDED TO INITIAL RA; BUT HAVE BEEN 'DESENSITIZED' IN THE TERMINAL AREA DUE TO HIGH VOLUMES OF TFC. SUPPLEMENTAL INFO FROM ACN 484895: ON APCH TO RWY 9R ATL OUTSIDE TIZZY INTXN AND CLRED VISUAL BEHIND COMPANY ACFT TOLD TO FOLLOW. LEVEL AT 3500 FT WE RECEIVED TCASII RA TO DSND. I SAW TFC ON TCASII WHICH WAS BEHIND AND TO OUR L 100 FT ABOVE. I LOOKED OUT TO OUR L AND SAW AN MD88 TURNING ONTO RWY 8L APCH PATH AND THOUGHT THIS WAS CAUSE OF RA. AS THE OTHER MD88 LEVELED OFF; THE TCASII STOPPED AND WE FIGURED WE WERE CLR. TCASII AGAIN STARTED WITH DSCNT COMMAND; WHICH WE BEGAN TO FOLLOW. WE ADVISED APCH WHO THEN TOLD US OF VFR TFC AT 3500 FT. OTHER ACFT (UNKNOWN) RECEIVED RA TO CLB. DON'T KNOW HOW CLOSE THE VFR WAS BUT PROBABLY A COUPLE OF MI BUT ONLY 100 FT ABOVE. CALLED ATL APCH AND TALKED TO APCH DESK. THEY SAID THEY WERE TRACKING VFR ACFT THAT HAD VIOLATED CLASS B AIRSPACE AND ASKED IF WE HAD BEEN TOLD OF TFC FIRST. I SAID NO. THEY SAID ACFT HAD NO XPONDER; BUT WE SAW TCASII RETURN WITH ALT. WE COULD HAVE BEEN GETTING 2 RA'S -- ONE FOR MD88 AND OTHER FOR VFR. THEY WERE SO CLOSE AND TCASII SCREEN IS NOT THAT ACCURATE OR DETAILED AT 10 MI RANGE SELECTED.