DC10 CREW WAS REQUIRED TO SHUT DOWN #3 ENG DUE TO LOSS OF ENG OIL. THEY HAD PREVIOUSLY DISCONNECTED #2 CSD. AN EMER WAS DECLARED AND A DSCNT AND DIVERSION WAS BEGUN. SINCE THEY WERE ON SINGLE GENERATOR OP; AN APU START WAS ATTEMPTED AT FL250. IT WAS UNSUCCESSFUL UNTIL A DSCNT TO 10000 FT WAS MADE.

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SHUT DOWN OF #3 ENG DUE TO LOSS OF OIL. WAS UNABLE TO MAINTAIN FL360; SO DECLARED EMER; TURNED OFF NAT TRACK AND PARALLELED IT WITH A 30 NM OFFSET. THEN DSNDED TO FL250 IN HOPES OF STARTING THE APU. THE #2 CSD HAD BEEN PREVIOUSLY DISCONNECTED; SO STARTING THE APU WAS DESIRABLE; TO REGAIN A 2 GENERATOR OP. AS THE APU WOULD NOT START INITIALLY; WE STEPPED DOWN TO LOWER ALTS TO WARMER OUTSIDE AIR TEMPS. AT 10000 FT THE APU STARTED; THEN WE CLBED BACK UP TO FL200. SHANWICK RADIO WAS ADVISED OF EVERY ALT CHANGE. KEFLAVIK AND SHANNON WERE OUR DIVERSION CHOICES. KEFLAVIK WAS 80 NM CLOSER; HOWEVER; THE WX IN SHANNON WAS MUCH BETTER; SO WE RECEIVED A DIRECT CLRNC TO SHANNON. APPROX 1 HR 10 MINS LATER; WE ARRIVED SHANNON WITH NO FURTHER INCIDENT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: CREW WAS FLYING A DC10-40 WITH P&W JT-9D-20 ENGS EBOUND FROM IAD TO INN. THE CREW WAS REQUIRED TO DISCONNECT #2 CSD DUE TO THE CSD LOW OIL PRESSURE LIGHT ILLUMINATING. LATER IN THE FLT; THE #3 ENG OIL FILTER BYPASS LIGHT ILLUMINATED. THE ABNORMAL CHKLIST MANDATES AN ENG SHUTDOWN FOR THIS. THIS LEFT THE ACFT WITH ONLY 1 OPERATING GENERATOR; #1. THE APU STARTING ENVELOPE LIMIT IS FL250. AFTER DSNDING TO FL250; 2 APU START ATTEMPTS WERE MADE UNSUCCESSFULLY. THE CREW THEN CONTINUED DSCNT TO FL200. 2 MORE ATTEMPTED STARTS WERE UNSUCCESSFUL. DSCNT CONTINUED TO 15000 FT WHERE ANOTHER 2 ATTEMPTS WERE MADE. ONLY AFTER A DSCNT TO 10000 FT; AND AFTER ALLOWING THE APU TO WARM UP ABOUT 25 MINS WAS A SUCCESSFUL START ACCOMPLISHED. ALL IN ALL; IT TOOK 7 TRIES BEFORE THE START WAS SUCCESSFUL. THE CREW ALLOWED THE APU TO OPERATE FOR ABOUT 15 MINS BEFORE DOING A 2 ENG CLB BACK TO FL250. APU GENERATOR OP AFTER THE APU WAS STARTED WAS NORMAL. THE PLT STATED HIS COMPANY DOES NOT TEST THE APU'S TO SEE IF THEY WILL OPERATE AT THEIR STARTING ENVELOPE LIMITS.