A319 FLT CREW ON DOGLEG TO FINAL AT PHL TURNS AWAY FROM INTERCEPT; CAUSING EVENTUAL LOSS OF SEPARATION WITH THIRD ACFT ON FINAL BEHIND RPTR.

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AFTER A LONG DUTY DAY; AND HOLDING AT VCN; WAS VECTORED FOR ILS RWY 27R IN PHL. WHEN ON AN INTERCEPT HDG AND CLRED FOR THE APCH; WHILE DSNDING THROUGH 3500 FT FOR 3000 FT; UPON LOC CAPTURE THE AIRPLANE STARTED A R TURN (INSTEAD OF L). I TOOK CTL OF THE ACFT; TURNED OFF THE AUTOPLT AND FLT DIRECTOR AND MANUALLY CONTINUED DOWN TO 3000 FT AND REVERSED THE TURN BACK TO THE L TO INTERCEPT FINAL APCH COURSE. RECEIVED AND COMPLIED WITH RA TO MONITOR VERT SPD. COMPLIED WITH RADAR VECTORS IN BOX PATTERN TO RE-INTERCEPT THE APCH GAVE THE ACFT TO THE FO WHILE I REPROGRAMMED THE FMGC. THE APCH AND LNDG WERE WITHOUT FAULT. AFTER A LONG DAY; WE WERE TAKEN BY SURPRISE WITH THE R TURN AND WERE MUCH TOO BUSY TO FIGURE OUT WHAT MAY HAVE GONE WRONG. MORE CONCERNED WITH WHERE WE WERE GOING NEXT. AFTER LNDG; CHKED THE SECOND FLT PLAN WHICH WAS COPIED FROM THE PRIMARY FLT PLAN PRIOR TO TKOF; IT WAS CORRECT. I DO NOT KNOW THE CAUSE FOR THE INCORRECT TURN AT LOC CAPTURE; BUT WHEN THINGS GO WRONG; THE COCKPIT GETS VERY BUSY AND THE AUTOMATION BECOMES MORE OF A DISTR FROM FLYING AND LOOKING FOR TFC THAN A HELP! SUPPLEMENTAL INFO FROM ACN 635674: RECEIVED AND ACKNOWLEDGED A DOGLEG TURN TO FINAL AND APCH CLRNC. WITHIN 1 MIN; ACFT TURNED COMPLETELY AWAY FROM FINAL APCH COURSE INTO THE PATH OF THE ACFT Y FOLLOWING HIM IN ON THE FINAL. BECAUSE OF HIS DEV; I WAS FORCED TO TURN ACR Y OUT OF HIS WAY; BUT INTO THE FLT PATH OF YET THE NEXT ARR. BECAUSE I WAS FORCED TO REACT TO PREVENT THIS COLLISION; MY ONLY OPTION WAS TO TURN ACR Y SBOUND. ADDITIONALLY; ACR X DSNDED THROUGH HIS ASSIGNED ALT AND REQUIRED EVEN FURTHER INSTRUCTIONS FROM ME. I HAD NO IDEA WHAT THIS ACFT WAS DOING; WOULD DO AND IT REQUIRED ALL OF MY ATTN; LEAVING NONE FOR THE 6 OTHER ACFT. OBVIOUSLY; THIS CREW NEEDED TO NOT ONLY PAY ATTN TO INSTRUCTIONS; BUT BE FAMILIAR ENOUGH WITH THE ENVIRONMENT TO KNOW THAT A NEBOUND HDG WOULD NOT BE THE DIRECTION TO THE ARPT.