A CRJ-200 FLT CREW EXPERIENCED EICAS MESSAGE 'HYD SYS 3 TEMP.' THEY RAN THE QRH; DECLARED AN EMERGENCY; AND DIVERTED TO THE NEAREST SUITABLE AIRPORT.

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THE FLT BEGAN WITH A DEFERMENT OF THE HYD SYS 3A PUMP DUE TO IT CAUSING EXCESSIVE HEATING OF THE HYD FLUID. AFTER REVIEWING THE MEL FOR THE DEFERRED EQUIP AND CONSULTING WITH MAINT CTL ON THE SITUATION AND PROPER PROC FOR CONTINUING THE FLT WE PROCEEDED. DURING NORMAL TAXI; TKOF; CLB AND TRANSITION TO CRUISE WE WATCHED THE HYD 3 SYS TEMP RISE STEADILY EXCEEDING THE TEMPS OF THE OTHER 2 SYS BY ALMOST 3 TIMES THE AMOUNT INDICATED. ALL OF THIS WAS CONFIRMED BY CONSULTING THE HYD SYNOPTIC PAGE ON OUR EICAS SYS. AT REACHING 97 DEG C THE AMBER ALERT MESSAGE APPEARED ON EICAS ONE DISPLAY INDICATING 'HYD SYS 3 TEMP.' UPON READING THIS WE INITIATED THE PROC FOR CONFIRMING THE MESSAGE; RESETTING THE MASTER CAUTION LIGHT AND CHKLIST IN THE POM. THE ALERT MESSAGE LED US TO THE HYD SYS HIGH TEMP CHKLIST. WE RAN THROUGH THE CHKLIST WHICH TOLD US TO TURN HYD 3B PUMP OFF WHICH ADDED TO THE ALREADY DEFERRED 3A PUMP BEING OFF. AFTER PLACING THE 3B PUMP IN THE OFF POS AN ACCOMPANIED ALERT MESSAGE SHOWED ON EICAS 1 DISPLAY. THE MESSAGES INDICATED 'HYD 3 LOW PRESS;' 'IND GND SPOIL.' THE LATTER OF THE TWO MESSAGES WAS ASSOCIATED WITH THE LOW PRESSURE MESSAGE. WE PROCEEDED TO RUN THE CHKLIST FOR THE LOW PRESSURE MESSAGE. THE MESSAGE STATED TO LAND AT THE NEAREST SUITABLE ARPT. WITH OVER 46 MINS TO GO TO ZZZ1 AND ZZZ BEING 17 MINS AWAY THE DECISION WAS MADE TO UTILIZE THE LOW AND DIRECT HEADWINDS AT ZZZ; AS WELL AS THEIR LONG RWY. WE REVIEWED THE LOST SYS; ADDED RESTR TO LNDG AND OUR EXISTING REQUIREMENTS AND ASKED ATC FOR ROUTING FOR A DIVERSION TO ZZZ. UPON FURTHER REALIZING THE LOSS OF NOSEWHEEL STEERING; THE ABILITY TO MANIPULATE THE LNDG GEAR SYS NORMALLY AND LOSS OF A SPOILER SYS AN EMER WAS DECLARED. THE LNDG WAS UNEVENTFUL WITH NO STRUCTURAL DAMAGE TO THE AIRFRAME AND NO PERSONAL INJURY TO CREW; PAX OR GND CREW AND ARPT PROPERTY. A COMMENDABLE USAGE OF CRM TAUGHT BY ACR WAS STRICTLY UTILIZED TO THE POSITIVE OUTCOME OF THIS PREDICAMENT. FROM THE USAGE OF CHKLIST; DIVISION OF TASKS AND THOROUGH USE OF EMER PROCS THE FLT LANDED SAFELY WHERE WE WERE MET BY ARFF SVCS. THE SUPPORT FROM EVERYONE INVOLVED I BELIEVE IS ONE OF THE REASONS WE WERE SUCCESSFUL IN THIS DIVERSION. AFTER CONSULTING MAINT ON THE VIABILITY OF THE DEFERRAL ON THE 3A PUMP AND BEING TOLD TO PROCEED AS DIRECTED BY THE MEL. (WE WERE UNSURE BASED OFF THE ORIGINAL WRITE-UP AND THE MEL) WE TOOK IT UPON OURSELVES TO CONTINUOUSLY MONITOR THE HYD PAGE FOR THE TEMP AND PRESSURE. WE OBSERVED IT CONTINUOUSLY RISING TO THE POINT THE CAUTION MESSAGE APPEARED. WE RAN THE HIGH TEMP CHKLIST AND THEN THE LOW PRESSURE CHKLIST. I BELIEVE THE EVENT OCCURRED DUE TO THE WRITE-UP AND DEFERRAL OF THE HYD 3A PUMP. A NOTE WAS IN THE WRITE-UP ALSO STATING THAT USING THE 3B PUMP THE HYD FLUID STILL HEATED UP PAST LIMITS. THE ACFT SHOULD HAVE BEEN FIXED IN ZZZ3 THE ORIGINATION OF THE DEFERRAL. CAREFULLY REVIEW THE WRITE-UP AND USE BETTER JUDGEMENT ON WHAT IS SAID. IF 3A PUMP HEATS THE FLUID SO OPTION IS USE 3B PUMP BUT THAT HEATS THE FLUID DO NOT USE MAINT DEFERRALS TO CONTINUE THE USE OF THE ACFT FOR PURE OP REASONS. COMMON SENSE IN THIS CASE WOULD HAVE DICTATED BEING THE WRITE-UP WAS IN ZZZ3 AND THE PART WAS IN ZZZ3 TO REMOVE THE ACFT FROM SVC AND FIX IT THEN RETURN IT TO SVC NOT ALLOW IT TO MAKE 3 OR 4 MORE TRIPS. I DO NOT CITE THE CREW THAT WROTE IT UP; MAINT OR DISPATCH ON THIS PROB BUT BELIEVE IT WAS JUST A MISCALCULATION.