ACR B747 AFTER MEETING A DSCNT XING RESTR DSNDS BELOW ASSIGNED ALT WHILE MAKING PWR ADJUSTMENT TO #3 ENG.

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WHILE FLYING AS FO AS THE PF FROM LAX TO JFK; WE EXPERIENCED AN ALTDEV. WE WERE FLYING THE LENDY 4 ARR TO JFK. ATC CLRED US TO CROSS HARTY INTXN AT FL230. I MADE THE DSCNT FROM CRUISE ALT USING THE FLT DIRECTOR BUT NO AUTOPLT. WE DSNDED AT THE PROPER TIME AND MADE THE XING RESTR WITHOUT A PROB. I INITIATED THE LEVELOFF AS WE APCHED FL230 USING THE FLT DIRECTOR BUT STILL NO AUTOPLT. AS I LEVELED OFF I BEGAN TO INCREASE PWR. AT THAT TIME I NOTICED THAT THE #3 ENG WAS SLOW TO RESPOND. DUE TO A SLIGHTLY DIFFERENT RIGGING FROM THE OTHER ENGS; THIS THROTTLE NEEDED TO BE PUSHED UP FURTHER THAN THE OTHER 3 IN ORDER TO PRODUCE THE SAME PWR. AS I WAS MAKING THIS PWR ADJUSTMENT I GLANCED BACK OVER TO MY FLT INSTS AND NOTICED THAT I HAD DSNDED TO FL225. AT THIS SAME TIME THE CAPT NOTICED MY ALT AND ADVISED ME TO 'WATCH THE ALT.' I IMMEDIATELY PULLED THE NOSE UP AND QUICKLY GOT BACK TO FL230. AT THAT TIME ATC CALLED US TO 'SAY YOUR ALT.' SINCE WE WERE NOW BACK AT FL230 WE REPLIED 'FL230.' THE CTLR THEN TOLD US TO 'DISREGARD.' THE REMAINDER OF THE FLT WAS UNEVENTFUL. THIS DEV WAS CAUSED BY MY ALLOWING MYSELF TO BECOME DISTR BY THE #3 THROTTLE AND SUBSEQUENTLY ALLOWING THE ALT TO DEV. HAD I USED THE AUTOPLT WHILE I WAS ADJUSTING THE THROTTLE THIS ALTDEV WOULD HAVE BEEN PREVENTED. ALSO; HAD EITHER THE CAPT OR THE FE'S BROUGHT THE ALT TO MY ATTN SOONER; THIS MIGHT HAVE BEEN AVOIDED.