Connection of firewire to thunderbolt 3

You need to buy 2 adapters:

1) Apple Thunderbolt 3 to Thunderbolt

2) Apple Thunderbolt to FireWire

Depending on your Iomega drive you will either have a FireWire400 oder FireWire800 connector. The adapter mentioned above gives you a FireWire800 port. If your Iomega drive has that you can just connect your FW800 cable to it. If it does only FW400 you can get an FW400 to FW800 adapter as well, but in this case I would suggest to get a FW400 to FW800 cable instead.

The chain will look like this: iMac -> TB3 to TB -> TB to FW -> FW cable to device

What model # is that Iomega external hard drive?

Many of those old FireWire (Iomega, Maxtor, LaCie, OWC, etc..) external drives came with optional FireWire/USB port connections and cables.

If that external drive also has USB port, then all you need to do is find the original USB cable and use an inexpensive USB-C to USB-A adapter.

If the old Iomega drive has a USB port that looks like the one in the red box?

Then you need a USB cable that looks like the following one.

And a USB-C to USB-A adapter that looks the following one.

The "printer" output, I assume a DB-25, is for parallel port.

You might be able to open it up, pull out the drive and get an IDE* to USB adapter. I did that with a dead firewire drive.

*Or whatever

You're welcome.