A B727 FREIGHTER DSNDS FROM 3000 FT TOO SOON ON AN ALT OVERSHOOT IN A NIGHT OP.

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ON VECTORS TO INTERCEPT FINAL APCH COURSE FOR ILS APCH TO RWY 17 AT MHT; WE WERE ASSIGNED 3000 FT MSL. AFTER PASSING 4000 FT MSL; I NOTICED THAT MY GS FLAGS WERE IN VIEW. (I WAS FLYING THE LEG.) I MENTIONED THE FLAGS TO THE CAPT. SINCE WE HAD BRIEFED THE FULL ILS APCH; BUT NOT A LOC APCH; WE BOTH STARTED ANALYZING THE APCH PLATE TO DETERMINE OUR NEW MINIMUMS AND VDP (VISUAL DSCNT POINT). WHEN I LOOKED UP TO CHK OUR ALT; WE WERE AT 2700 FT MSL AND DSNDING. I TOOK CORRECTIVE ACTION TO CLB BACK TO 3000 FT. AT THE SAME TIME THAT WE NOTICED OUR ALTDEV; THE APCH CTLR NOTICED IT; AND SHE THEN CLRED US TO A LOWER ALT. SHE DID NOT SEEM PARTICULARLY BOTHERED BY THE DEV; SINCE THERE WAS NO OTHER TFC IN THE AREA. CONTRIBUTING FACTORS INCLUDE: 1) NOT ANTICIPATING THE POSSIBILITY THAT THE GS WOULD BE INOP; AND NOT BRIEFING THE LOC APCH AHEAD OF TIME. 2) BOTH THE FO AND CAPT FOCUSING ON THE APCH PLATES AT THE SAME TIME; WITH NO ONE 'WATCHING THE AIRPLANE.' SUPPLEMENTAL INFO FROM ACN 408531: FO WAS FLYING ACFT DSNDING TO 3000 FT ON VECTORS FOR AN ILS APCH. AT THE TIME I WAS DISCUSSING WITH MHT APCH THE LACK OF ILS DME INFO FROM THE RWY 17 ILS. THE ALTDEV CHIME SOUNDED WE HAD PASSED 3000 FT DOWN TO 2700 FT. CONTRIBUTING FACTORS: SHORT FLT FROM PWM; WITH MANY ITEMS TO RESPOND TO ON THE 'IN RANGE' AND 'APCH' CHKLISTS. WE WERE ACCOMPLISHING THESE CHKLISTS AT THE TIME OF THE OCCURRENCE.