DURING CLB; A B727 FLC NOTICED A FUEL SYS PROB AFTER A FUEL IMBAL BTWN THE #1 AND #3 TANKS HAD DEVELOPED. IT WAS DETERMINED THAT THERE WERE APPROX 18000 LBS OF UNUSABLE FUEL ONBOARD; SO THE FLT DIVERTED. THE MECH'S RPT INDICATES THAT PRIOR TO DEP HE HAD INADVERTENTLY SECURED THE #1 XFEED VALVE IN THE OPEN POS AS OPPOSED TO THE #3 VALVE; THAT WAS INOP AND FAILED IN THE CLOSED POS.

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THE FLT WAS SCHEDULED FROM ZZZ TO XXX. THE ACFT WAS AN ADVANCED B727. TOTAL FUEL LOAD AT THE GATE PRIOR TO DEP WAS 52600 LBS. WITH THIS FUEL LOAD ALL XFEEDS WOULD BE OPENED FOR TKOF. WHILE PERFORMING THE FUEL XFEED TEST AND SET PROCS IN THE SO COCKPIT PREPARATION; THE #1 AND #2 XFEED VALVE WERE TESTED AND SET TO THE OPEN POS. HOWEVER; THE #3 XFEED VALVE IN TRANSIT LIGHT DID NOT COME ON AFTER ROTATION OF THE #3 XFEED VALVE SELECTOR. THE CAPT WAS INFORMED AND UPON INSPECTION OF THE CIRCUIT BREAKERS; THE #3 XFEED CIRCUIT BREAKER WAS OBSERVED TO BE POPPED AND WOULD NOT RESET. STATION MAINT WAS THEN NOTIFIED OF THE DISCREPANCY. AFTER INSPECTION; THE MAINT MEN INFORMED THE FLT DECK CREW THAT THE #3 XFEED VALVE WOULD BE MANUALLY OPENED AND THE PWR REMOVED. THE DISCREPANCY WAS TO BE DEFERRED AND UPDATED MAINT PAPERWORK WOULD BE PROVIDED. THE #3 XFEED CIRCUIT BREAKER WAS CAPPED AND AN ORANGE DEFERRED STICKER WAS PLACED BY THE #3 XFEED VALVE SELECTOR. THE MAINT PAPERWORK ARRIVED AND INDICATED THAT THE #3 XFEED VALVE WAS OPENED AND THE #3 FUEL SHUTOFF VALVE IN TRANSIT LIGHT WAS INOP. THIS DID NOT MAKE SENSE AND I BROUGHT IT TO THE ATTN OF THE CAPT. THE CAPT CHKED THE OP OF THE #3 FUEL SHUTOFF VALVE IN TRANSIT LIGHT WHICH WAS WORKING NORMALLY. HE THEN REQUESTED MAINT TO REVIEW THE ACTION AND RETURN WITH UPDATED MAINT PAPERWORK. AFTER A FEW MINS THE UPDATED MAINT PAPERWORK WAS BROUGHT TO THE FLT DECK WHICH INDICATED THE #3 XFEED VALVE WAS OPENED AND THE #3 XFEED VALVE IN TRANSIT LIGHT WAS INOP. TAXI AND TKOF WERE UNEVENTFUL. AFTER COMPLETING THE AFTER TKOF CHKLIST; I WENT TO SECURE THE #1 AND #3 FORWARD BOOST PUMPS. AT THIS TIME I NOTICED THAT THE FUEL IN TANK #3 WAS BEING DEPLETED WHILE TANK #1 REMAINED FULL. I NOTIFIED THE CAPT AND RECOMMENDED WE CLOSE THE #1 XFEED VALVE TO PREVENT THE IMBAL FROM GETTING OUT OF LIMITS. HE AGREED. UPON ROTATION OF THE #1 XFEED VALVE SELECTOR; THE #1 XFEED VALVE IN TRANSIT LIGHT DID NOT COME ON. THE FUEL IN THE #1 TANK WAS ALSO NOT BEING DEPLETED. THE CAPT AND I AGREED THAT THE #1 XFEED VALVE HAD FAILED IN THE OPEN POS. WITH THE #1 XFEED VALVE OPENED AND THE #3 XFEED VALVE CLOSED; THE CAPT AND I AGREED THAT THE #2 FUEL BOOST PUMPS SHOULD BE SECURED TO EVEN OUT THE #1 AND #3 FUEL TANK QUANTITIES. WITH THE FUEL IMBAL SIT UNDER CTL; THE FLC AGREED THAT THE ACFT HAD APPROX 18000 LBS OF UNUSABLE FUEL ON BOARD. AFTER DISCUSSION WITH MAINT AND DISPATCH IT WAS AGREED THAT THE FLT SHOULD DIVERT TO XXX. THE LNDG WAS UNEVENTFUL AND THE CAPT DEEMED IT AS SAFE TO LAND OVERWT; AS TO DUMP FUEL. TOTAL FUEL UPON LNDG WAS 30400 LBS; 4000 LBS; 22400 LBS; AND 4000 LBS RESPECTIVELY. THE RECENT TRAINING THAT I HAD RECEIVED OF 'ACTION: REACTION;' IE; WHEN A VALVE OR SWITCH IS MOVED LOOK FOR A POSITIVE VERIFICATION; PROVED TO BE INVALUABLE IN THAT IT ALLOWED ME TO ACCURATELY ASSESS WHAT THE PROB WAS AND PREVENT A FUEL IMBAL FROM OCCURRING. SUPPLEMENTAL INFO FROM ACN 337834: MAINT WAS CALLED TO ADDRESS A PROB. THE #3 FUEL XFEED VALVE CIRCUIT BREAKER COULD NOT BE SET. I HAD TO INOP THE #3 FUEL XFEED VALVE PER THE MEL. I INADVERTENTLY INOP'ED THE WRONG VALVE CAUSING A FUEL MGMNT PROB.