CAPT RPTS CONFUSION WITH APCH OVER RWY ASSIGNMENT FOR A VISUAL APCH; AND ENDS UP CLOSELY SPACED BEHIND ANOTHER ACFT. A GAR ENSUES.

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INBOUND ON THE ARR WE BECAME [AWARE] OF AN ATIS UPDATE THAT CHANGED THE RWYS FROM N LNDG TO S LNDG. THE COPLT WAS FLYING SO I CHANGED THE FMS TO A RWY 18R APCH AND THEN BRIEFED/RE-SET MINIMUMS FOR RWY 18R APCH. UPON CONTACTING APCH; WE WERE TOLD TO EXPECT RWY 18L FOR LNDG. I RELOADED FMS TO A RWY 18L APCH; BRIEFED RWY 18L; RESET MINIMUMS FOR A RWY 18L APCH. THERE WAS A LINE OF WX JUST N OF THE S APCHS SO WE MONITORED THIS ALSO. ON DOWNWIND; JUST N OF THE ARPT; WE WERE CHANGED TO ANOTHER APCH CTLR. AS HE VECTORED US TO BASE TURN WE CONFIRMED WE HAD THE FIELD IN SIGHT AND HE CLRED US FOR A VISUAL TO RWY 18L AND I READ BACK 'RWY 18L CLRED FOR THE VISUAL.' THE COPLT UNDERSTOOD THE SAME INSTRUCTIONS I DID. AS WE LINED UP ON FINAL; WE BECAME AWARE OF ANOTHER ACFT ON FINAL APPROX 1.5 MI AHEAD AND 1000 FT BELOW. DUE TO THE 3 PARALLEL RWYS WE FLEW FINAL FOR APPROX 20 SECONDS BEFORE WE DETERMINED HE WAS ALSO ALIGNED WITH RWY 18L AND WE INITIATED A TURN EBOUND OFF FINAL AND A CLB FROM APPROX 3400-4000 FT AND ANNOUNCED IT TO APCH CTL OVER THE RADIO. UPON HEARING THIS; THE CTLR BEGAN ISSUING VECTORS AND FOR THE FIRST TIME SAID THE WORDS 'RWY CHANGE' FOR RWY 18L. THIS ALERTED US BECAUSE WE HAD CONSIDERED RWY 18L THE PRIMARY RWY FROM INTXN XXXXX. WE WERE TOLD UPON INITIAL CONTACT TO EXPECT RWY 18L FOR LNDG. AT NO TIME WAS THERE A TCAS RA OR TA; AND WE HAD INITIATED THE GAR. UPON LNDG; I CALLED ATC OVER THE PHONE AND TALKED TO A SUPVR WHO HAD REVIEWED THE TAPE. HE TOLD ME THE CTLR HAD FAILED TO INFORM US OF THE RWY CHANGE FROM RWY 18L BUT HAD CLRED US FOR A VISUAL APCH TO RWY 18R AND WE HAD READ BACK RWY 18L AND INITIATED AN APCH TO RWY 18L. WE WERE IN A HIGH WORKLOAD ENVIRONMENT IN AN FMS AIRPLANE. THE MANY RWY CHANGES; WX; AND FAILURE TO USE THE WORDS 'RWY CHANGE' BY THE FINAL APCH CTLR WHO CLRED US FOR RWY 18R AFTER BEING TOLD TO EXPECT RWY 18L; ALL CONTRIBUTED TO OUR MISCOM.