I would say you have Zero chance of getting a 3Tb to work with your Dish-Tv S8100-ZA these STB's only support Fat32 and 3tb is way off the limitations of Fat32 .
There's also another problem with USB Powered HDDs as on most STBs they often just don't have enough current capability to power them satisfactorily. It's always advisable to use external power for external USB HDDs unless the box it's plugged in to is definitely known to have sufficient power to run them.
OK so power is not a problem but FAT32 only supports up to 2tb basic drives. You'll need to partition the drive to 2tb max. I don't know if the STB will support 2 x 1.5tb partitions on a USB HDD. They all seem to be rather fussy on what's plugged in and will work on USB.