A CRJ900'S #3 HYD SYSTEM HI TEMP EICAS MSG ALERTED IN FLT WITH THE 3A HYD PUMP DEFERRED. AN EMER WAS DECLARED AND THE FLT DIVERTED TO A NEARBY ARPT.

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WE WERE IN CRUISE AT FL360; ABOUT 50 MINS INTO A 1:43 MIN FLT; HALFWAY THERE BTWN ZZZ1 AND ZZZ2. WE RECEIVED A HYD 3 HI TEMP MESSAGE ON THE EICAS AND FOLLOWED THE POM CHKLISTS FROM THERE. WE WERE OPERATING WITH PUMP 3A DEFERRED; SO WITH HAVING TO TURN OFF PUMP 3B; WE HAD LOST A HYD SYS AND SOME NECESSARY HYD SVCS; LIKE LNDG GEAR EXTENSION; NOSEWHEEL STEERING AND INBOARD GND SPOILERS. THE ASSOCIATED CHKLISTS LED US TO DECLARING AN EMER; DIVERTING TO ZZZ AND A 12000 FT RWY WITH FAVORABLE WINDS. WE TRIED TO ACARS DISPATCH; AND WE BRIEFED THE FLT ATTENDANTS FOR AN EMER AND AN UNLIKELY POSSIBILITY OF EVAC. WE PERFORMED AN EMER GEAR EXTENSION; AND LANDED WITHOUT INCIDENT WITH ARFF STANDING BY. THE STATION WAS UNABLE TO TOW US TO THE GATE AS WE REQUESTED. SO AFTER THE PAX HAD BEEN BUSED TO THE TERMINAL; UNDER MAINT GUIDANCE; AND AFTER THE #3 HYD SYS HAD COOLED; WE STARTED UP THE ENGS AND TAXIED THE ACFT TO THE RAMP. WHEN WE RECEIVED THE ACFT FOR FLT WE NOTED THAT HYD PUMP 3A WAS DEFERRED. WHEN WE READ THE MEL; IT WAS A LITTLE UNCLR AS TO HOW TO OPERATE THE ACFT IN THAT CONDITION AS THE USUAL OP IS WITH 3A ON; AND 3B AUTO TO BACK UP 3A OR COME ON DURING VARIOUS FLT REGIMES FOR REDUNDANCY. MAINT SAID THAT 3B JUST STAYS ON IN THIS INSTANCE IN PLACE OF 3A. IN REVIEWING THE ORIGINAL WRITE-UP THAT LED TO HYD PUMP 3A BEING DEFERRED (HYD SYS 3 HAD EXPERIENCED A HI TEMP EARLIER THAT DAY ALREADY); IT APPEARED THAT IN THE PREVIOUS EVENT THE PROB MAY NOT HAVE BEEN ADEQUATELY ADDRESSED. THE WRITE-UP SAID THAT AFTER PUMP 3A CAUSED AN OVER-TEMP THE CREW HAD SWITCHED TO PUMP 3B; AND THOUGH THE SYS HAD COOLED SOME WITH THE PUMPS BOTH OFF; PUMP 3B WAS CAUSING THE SYS TEMP TO RISE ALSO. WE REALLY HAVE NO WAY TO KNOW IF THIS WAS ANYTHING OTHER THAN AN INCIDENT THAT HAPPENED TO OCCUR. ALTHOUGH IT APPEARED THAT BOTH PUMP 3A AND PUMP 3B WERE CAUSING THE TEMP IN THE #3 HYD SYS TO RISE AT THE TIME OF THE ORIGINAL WRITE-UP. IT COULD EASILY HAVE BEEN THAT THE MEL WAS CORRECTLY ASSIGNED AND THE ASSOCIATED CONDITION DETERIORATED EVEN FASTER THAN ANTICIPATED. OR IT MAY HAVE BEEN A MISDIAGNOSED PROB THAT COULD HAVE BEEN HANDLED MORE EFFECTIVELY IN ANOTHER WAY.