SCPI software developing this is fun and you do feel excitement and even pulling of your own hairs this is part of the job.
FETCH3? command this is FLUKE proprietary and special command this having the benefit of faster execution due 8846A DMM.
But does all 8846A units accept and respond to it?
The smoking gun when FETCH3? this is not executed this is the firmware version of 8845/8846 DMM it self.
Any multimeter loaded with 2008 firmware and older, this will not be capable to execute this command.
In the positive side of things, you are still able to use FETCH? or FETCH1? or FETCH2?, but for FETCH3? you need to contact FLUKE and request a copy of 2011 developed Firmware.
Firmware installation requiring LAN connectivity.
For the many out there I will recommend of you making an arrangement with your local FLUKE authorized reseller, so the upgrade to be performed from FLUKE, by this way they will get your money and the responsibility, about them delivering back your DMM undamaged.
The 8846A this is not equal to a cheap Pentium motherboard this requiring a minor BIOS firmware update.