INTENDED TO FLY A CAT III-A APCH BUT WOUND UP FLYING A CAT I APCH.

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ON A CAT III-A APCH TO RWY 9R PHL; WE WERE VECTORED FOR APCH AND INTERCEPTED THE LOC ABOUT 1 1/2 MI OUTSIDE THE OM. AS WE INTERCEPTED THE LOC; AND SHORTLY THEREAFTER THE GS; I ADVISED THE CAPT THAT HE NEEDED TO ENGAGE THE SECOND AUTOPLT FOR THE CAT III-A APCH. WE INTERCEPTED THE GS BEFORE THIS WAS DONE AND THE SECOND AUTOPLT WAS LOCKED OUT. THE CAPT ELECTED TO DISENGAGE THE AUTOPLT AND TRY TO ENGAGE BOTH AUTOPLTS AGAIN. ONLY 1 WOULD ENGAGE. THE OTHER WAS LOCKED OUT. THE CAPT DISENGAGED THE AUTOPLT AGAIN AT THIS TIME AND HAND FLEW THE APCH. HE WAS L OF COURSE ABOUT A DOT; AND LOW ABOUT 2 DOTS ON THE GS. PHL TWR ADVISED A 'LOW ALT ALERT' AND THE CAPT CORRECTED AND FLEW THE REST OF THE APCH. CONDITIONS WERE RAPIDLY IMPROVING AND WE BROKE OUT AT ABOUT 250 FT AGL WITH ABOUT 3/4 OF A MI VISIBILITY. THE ACFT WAS LANDED UNEVENTFULLY. I FEEL WE WERE TURNED IN TOO CLOSE TO THE OM AND THIS PRECIPITATED A SIT WHERE THE SAFE OP OF THE FLT WAS COMPROMISED FOR A CAT III-A APCH. WE INTERCEPTED TOO CLOSE AT THE OM AT TOO GREAT AN ANGLE. WHILE MONITORING THE INTERCEPT; THE LOC AND GS SIMULTANEOUSLY; WE WERE DISTRACTED ENOUGH TO OVERLOOK THE ENGAGEMENT OF THE SECOND AUTOPLT CHANNEL; AND IT WAS DOWN HILL FROM THERE. IF IT WERE ACTUALLY CAT II OR III-A MINIMUMS; WE WOULD HAVE GONE AROUND AT CAT I MINIMUMS.