AN ARR C560 IS GIVEN 3 DIFFERENT APCHS TO STL WITHIN A 10 MI DISTANCE. THE PIC EXPERIENCED AN EXCURSION FROM THE ASSIGNED ALT WHEN HE GETS CONFUSED ABOUT THE DSCNT POINT WHICH WAS A RADAR FIX THAT WAS NOT IN THE FMC.

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MY CO-CAPT AND I WERE APCHING STL. CHKING THE ATIS; WE WERE EXPECTING AN ILS RWY 12L. WHEN WE CHKED IN WITH APCH WE WERE ADVISED TO EXPECT THE LDA RWY 12L APCH. AT THIS TIME WE HAD ALREADY BRIEFED THE ILS RWY 12L; SO WE REVIEWED THE NEW APCH. WHEN WE WERE TURNED OVER TO THE FINAL CTLR HE INSTRUCTED US TO EXPECT THE RWY 12R ILS. WE WERE NOW IN CLOSE TO THE ARPT AND AGAIN HAD TO BRIEF A NEW APCH; WHICH WE DID. WE WERE THEN INSTRUCTED TO MAINTAIN 4500 FT TILL PAULY INTXN CLRED ILS RWY 12R; CONTACT THE TWR. PAULY INTXN WAS A RADAR FIX AND WE DID NOT HAVE IT PROGRAMMED INTO THE FMS; SINCE I WAS MENTALLY EXPECTING A CLRNC TO A LOWER INITIAL APCH ALT. IN THE RUSH TO IDENT PAULY INTXN I BELIEVE I MAY HAVE STARTED DOWN PREMATURELY BY 1 MI OR SO; BUT RECAPTURED THE GS AND CONTINUED THE APCH. BRIEFING 3 DIFFERENT APCHS IN A SHORT PERIOD OF TIME AND NOT HAVING A GOOD GRASP OF THE APCH WE WERE FLYING LED TO CONFUSION IN THE COCKPIT. I SHOULD HAVE REQUESTED A HOLD OVER THE INTXN AND LOOKED OVER THE PLATE THEREBY KEEPING MY TOTAL FOCUS ON THE APCH.