C182 ON VISUAL APCH TO PDK EXPERIENCED NMAC WITH TFC TURNING FINAL AHEAD AT APPROX 800 FT.

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I WAS WITH MY WIFE ON AN IFR FLT TO PDK ARPT. I WAS RADAR VECTORED (BY ATL APCH) TO THE NE OF PDK AND TOLD TO EXPECT THE VISUAL TO RWY 20L. AT APPROX 8 MI FROM THE ARPT; I WAS TOLD TO CONTACT THE TWR. ON MY SECOND ATTEMPT; THE TWR ACKNOWLEDGED ME AND I WAS TOLD TO 'PROCEED.' SHORTLY THEREAFTER; I WAS TOLD THAT I WAS '#3 FOR RWY 20L.' AT THAT DISTANCE; I COULD NOT SEE THE RWY ENVIRONMENT WELL; SO I INTERCEPTED THE LOC TO RWY 20L AND TRACKED IT INBOUND. BY RADIO COM; IT WAS OBVIOUS THAT THERE WERE MULTIPLE PLANES IN THE PATTERN; BOTH RWY 20L&R WERE IN USE AND THAT THE CTLR WAS VERY BUSY. I WAS GIVEN NO FURTHER INFO FROM THE TWR (DISPATCH) ABOUT THE IDENT AND LOCATION OF THE #1 AND #2 ACFT. I ASSUMED THAT THEY WERE IN THE PATTERN; L TFC; BUT I COULD NOT SEE THEM. SOMETIME THEREAFTER; I SAW A PLANE ON SHORT FINAL FOR RWY 20L. I ASSUMED THAT IT WAS THE '#2 PLANE' AND THAT THE '#1 PLANE' HAD LANDED. IN SHORT ORDER; I BEGAN TO WORRY ABOUT ASSUMPTION. AT THE POINT THAT I WAS ABOUT TO QUERY THE TWR; (AT LESS THAN 1 MI FINAL; 800 FT AGL) A C152 TURNED INTO MY FLT PATH FROM A L BASE TO FINAL APCH. WE WERE SO CLOSE THAT I COULD SEE THE RIVETS ON HIS FUSELAGE. THIS NECESSITATED A HARD R DSNDING BANK THAT PUT ME (IT SEEMED RATHER QUICKLY) INTO THE APCH COURSE FOR RWY 20R. ONCE I WAS SUFFICIENTLY BELOW AND BEHIND HIM; I SHOUTED 'WHOA' INTO THE MIKE AND MADE A L TURN TO THE SE OUT OF THE RWY 20R COURSE. I THEN SAID 'TWR; ACFT X.' HE ASKED IF I WAS NOW BEHIND THE C152; TO WHICH I RESPONDED IN THE AFFIRMATIVE. HE THEN DIRECTED ME TO DO A L 280/360 DEG TURN AND CLRED ME TO LAND ON RWY 20L. WE LANDED WITHOUT INCIDENT. I DO NOT BELIEVE THAT I WAS EVER SEEN BY THE C152. MY EVASIVE MANEUVER CERTAINLY CAUSED A THIRD ACFT ON THE APCH TO RWY 20R TO DO A GAR. MY PERCEPTION IS THAT I DID AN INADEQUATE JOB OF VISUALIZING THE #1 AND #2 ACFT AND SHOULD HAVE QUERIED THE TWR EARLIER ABOUT THEIR POS. I WAS LULLED BY THE FACT THAT I WAS ON AN IFR FLT PLAN WITHIN AND THEN UNDER CLASS B AIRSPACE AND WAS RADAR CONTACT. I REALIZE THAT IT IS THE PLT'S RESPONSIBILITY TO SEE AND AVOID OTHER ACFT WHILE IFR IN A VFR ENVIRONMENT. IT IS ALSO INCREDIBLE TO ME THAT THIS COULD BE ALLOWED TO HAPPEN IN A RADAR ENVIRONMENT AND I FEEL THAT THE ACFT SPACING WAS OBVIOUSLY INADEQUATE.