If you would like to remove the DVD drive from the Iomega USB enclosure and install it in your B&W G3's optical drive bay, yes - it can be done. In recent years, Iomega has installed standard 5.25" ATAPI burners (CD-R/RW or DVD±RW) in their external cases, often using "Lite-On" brand drives. You may or may not be able to boot the G3 from the optical drive, but Lite-On drives have better Apple-compatibility than some other third-party brands. That Iomega enclosure has no visible means of fasteners/screws that attach the top cover to the base. Instead, you must insert a small tool into (2) locations on both sides where the upper and lower pieces meet, to depress the internal latches and separate them. The drive should already be factory-set to Master, which is exactly how it should be configured for your B&W. Unless you already have the Iomega Super DVD drive, it would be less expensive to just buy/install a 5.25" internal DVD±RW drive - like this
Pioneer burner.