C172 RPTR HAS NMAC WITH AN SMA ON SHORT FINAL APCH.

Narrative

I WAS NEARING THE END OF A VFR FLT FROM LINCOLN ARPT; CA; AT 2500 FT MSL WHEN I WAS CHANGED FROM TRAVIS APCH CTL TO CONCORD TWR FREQ 119.7. I RPTED TO CONCORD TWR 2 MI N OF PORT CHICAGO (6-7 MI FORM CCR) WITH INFO 'TANGO.' I WAS CLRED TO ENTER R TFC FOR RWY 32R AND WAS TOLD THAT THERE WAS A CESSNA ON AN INSTRUMENT APCH TO RWY 19 TO CIRCLE L FOR LNDG ON RWY 32R AND THERE WAS ALSO ANOTHER CESSNA ON AN INSTRUMENT APCH BEHIND ME AND WAS STILL TALKING TO TRAVIS APCH. I RPTED THE CESSNA BEHIND ME WAS IN SIGHT. THERE WERE SEVERAL ACFT IN THE PATTERN AND I WAS WATCHING FOR THEM AS I DSNDED TO 1000 FT MSL. I ENTERED THE R DOWNWIND AT MID FIELD FOR RWY 32R AND CALLED 'DOWNWIND' AND WAS TOLD BY THE TWR CTLR THAT I WAS #4 TO LAND WITH 2 CHEROKEES IN FRONT OF ME AND THE ONE I WAS FOLLOWING WAS TURNING BASE NEAR MT DIABLO HOSPITAL. I PROCEEDED DOWNWIND AND TWR CLRED ME TO LAND BEHIND THE CHEROKEE THAT WAS TURNING FINAL. I LOOKED AND THERE WAS A LIGHT COLORED; LOW WING ACFT ROLLING OUT ON FINAL. I TOOK THAT TO BE THE ACFT I WAS TO FOLLOW AND I BEGAN A DSNDING TURN TO BASE NEAR THE HIGH RISE BUILDINGS IN DOWNTOWN CONCORD. AS I ENTERED BASE; THERE WAS 1 LOW WING ACFT TOUCHING DOWN AND 1 LOW WING ACFT ON SHORT FINAL AND A CESSNA TAXIING OFF RWY 32R. I ASSUMED THESE WERE THE 3 ACFT IN FRONT OF ME. WHILE ON BASE I CLRED FINAL AND DID NOT SEE ANY ACFT. I TURNED ONTO FINAL AND AS I WAS ABOUT TO ROLL OUT; I NOTICED THAT I WAS A LITTLE HIGH ON THE VASI SO I HELD IN A SLIGHT SLIP AT WHICH TIME I NOTICED A RED AND WHITE CHEROKEE ON MY LEFT; AT THE SAME ALT; AND CONVERGING. I THEN STARTED AN IMMEDIATE TURN TO THE R AND ADDED PWR TO STOP MY DSCNT AND START A CLB. AT ABOUT THIS TIME THE TWR CTLR TOLD THE CHEROKEE TO GAR AND/OR ME TO CONTINUE IN MY R TURN FOR A 360 DEG TURN AND RETURN FOR LNDG. A DIFFERENT CTLR CLRED ME TO LAND; AS I FOUND OUT LATER THEY WERE IN THE MIDST OF CHANGING POS IN THE TWR.