A B737-300 CREW CAT3 PROCEDURAL ERROR ALLOWED AN EARLY DESCENT; FOLLOWED BY A GS ALERT; AND A GO AROUND PRIOR TO INTERCEPTING THE GS.

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LEFT TO GO TO GEG. ON EARLIER LEG WE DISCUSSED THE PROBABLE CAT III APCH INTO GEG. AFTER DEPARTING; I BRIEFED CAT III INTO GEG (PROBABLY TOO EARLY). CLRED TO 4000 FT AND CLRED FOR THE APCH. AFTER JOINING; I SET 3000 FT INTO THE ALT WINDOW. IT SHOULD HAVE BEEN 3900 FT; AND SHOULD HAVE BEEN DONE BY THE PNF. WHY I DID THAT; I DO NOT KNOW. I WAS DIRTY; AND BELOW GS; BUT DSNDING TO 3000 FT TO LEVEL OFF AND THEN INTERCEPT GS. ALMOST SIMULTANEOUSLY; THE FO CALLED GS; THE GS ALERT WENT OFF; THE TWR CALLED LOW ALT ALERT; AND THEN WE GOT APCH WARNING. I SUBSEQUENTLY EXECUTED A GAR TO ANOTHER APCH. THE FO AND I SUBSEQUENTLY TRIED TO DO A RECALL OF EVENTS THAT LED TO THE BREAKDOWN. I CANNOT REMEMBER IF HE READ BACK AND ACKNOWLEDGED 3000 FT SET IN THE ALT WINDOW. I DO NOT KNOW WHY I SET 3000 FT IN THE WINDOW WHEN IT SHOULD HAVE BEEN 3900 FT. SUPPLEMENTAL INFO FROM ACN 725195: WHILE MAINTAINING 4000 FT WE WERE INTERCEPTING THE LOC AND GS. CAPT/PF CALLED FOR MCP ALT 3000 FT. I SET 3000 FT IN MCP. (NOT SURE OF THIS AS I WAS XCHKING MY APCH PLATE; THINKING THE CAPT/PF WAS SETTING A MISSED APCH ALT?) WE WERE DEVIATING FROM GS AND CONFIGURING TO FLAPS 40 DEGS. THE CAPT/PF CALLED FOR AIII MODE. I CALLED 'GS' AS WE DEVIATED FROM GLIDE PATH AND WE LEVELED OFF TO RE-INTERCEPT AT APPROX 3500 FT MSL AND 6.0 DME. (WE WERE ESTIMATED 350-400 FT BELOW GS.) AS THE CAPT ADDED PWR TO MAINTAIN ALT AND CORRECT TO GLIDE PATH; ACFT AIRSPD INCREASED TO APPROX 160 KIAS AND WE RECEIVED AN APCH WARNING AND I CALLED 'APCH WARNING; GO AROUND.' WE INITIATED A GAR; ADVISED ATC AND CLBED TO 4000 FT MSL. FAILURE TO MAINTAIN ALT AT 4000 FT MSL UNTIL INTERCEPTING GLIDE PATH (PUBLISHED 3900 FT MSL). CAPT PF REQUEST TO RESET MCP ALT WINDOW TO 3000 FT AND MY PLT MONITORING MOMENTARY CONFUSION WITH REQUEST (MISINTERPRETING HIS REQUEST AS A MISSED APCH ALT). MOVING MAP DISPLAY WOULD HAVE AIDED IN SITUATIONAL AWARENESS IN RELATION TO ARPT/APCH FIXES. HGS GUIDANCE REPEATED ON FO SIDE. CREW'S PROC FOR FLYING DIVE AND DRIVE APCHS LEADS TO CONSTANT DSCNT APCHS AND PROCS IN THE FOM FOR DECELERATING WHILE ON A 3 DEG GLIDE PATH (EG; 13 NM TO DECELERATE 250 KTS CLEAN TO CONFIGURED FLAPS 40 DEGS AT TARGET).