A B767 FLC CROSS THEIR ASSIGNED FIX 2500 FT BELOW THEIR ASSIGNED ALT ON THE CHINS 3 ARR AT SEA; WA.

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WE WERE LEVEL AT FL240; AND WERE CLRED TO DSND TO 12000 FT TO COMPLY WITH ALT RESTRS ON THE CHINS 3 ARR. THE FO HAD BRIEFED THE ARR AND HAD CORRECTLY ENTERED THE XING RESTRS INTO THE FMS; WHICH WERE TO CROSS RADDY INTXN AT 250 KTS; 1600 FT; AND DUBRN INTXN AT 250 KTS; 1200 FT. AFTER DSCNT CLRNC WAS GIVEN AND DSCNT WAS BEGUN; I WAS RECHKING INFO ON THE APCH PLATE FOR THE ILS TO RWY 16R. WHILE DOING SO; I HEARD THE FO MENTION THAT DRAG WAS REQUIRED TO MEET THE XING RESTR. SHORTLY AFTER THAT; I LOOKED UP TO SEE THAT THE FO HAD DESELECTED VNAV AND WAS IN FLT LEVEL CHANGE WITH SPD BRAKES EXTENDED. I REALIZED WE WERE DSNDING EARLY; AND BEFORE I COULD ASK HIM ABOUT THE DSCNT; ZSE SAID WE WERE TO CROSS RADDY AT 16000 FT. AT THE TIME; WE WERE AT APPROX 13500-14000 FT MSL. HE SAID THAT ALT RESTR WAS IN CASE TURBOPROPS WERE ON THE ARR; BUT THERE WERE NONE IN THE VICINITY; AND THERE WAS NO PROB. I ASKED THE FO ABOUT HIS DSCNT AND HE STATED ONCE WE WERE CLRED TO 12000 FT; HE INCORRECTLY 'LOCKED ON' TO THE 12000 FT AND WAS TRYING TO CROSS RADDY INTXN AT 12000 FT RATHER THAN THE REQUIRED 16000 FT. AFTER WE WERE SWITCHED TO APCH CTL; I AGAIN CONTACTED ZSE IN REGARDS TO THE EARLY DSCNT; AND HE SAID NO ONE ELSE WAS IN THIS AREA AND THERE WAS NOT A PROB. I BELIEVE FATIGUE WAS A FACTOR IN THIS INCIDENT AS WE ARE AN EAST COAST BASED CREW. WE HAD A 2 HR DELAY IN SLC PRIOR TO THIS LEG; AND I BELIEVE FATIGUE WAS A MAJOR CONTRIBUTING FACTOR. THE FO SAID AFTER ARR IN SEA THAT HE WAS VERY TIRED. ALSO; ALTHOUGH THE FO IS A VERY EXPERIENCED FO; I SHOULD KEEP A CLOSER WATCH ON A LONG DUTY DAY.