If there’s a USB controller in the dock attached over Thunderbolt, I’m pretty sure it’ll work; however if the USB 3 ports on the dock just use the inbuilt USB 3.1g2 bus on TB3 through some kind of hub, obviously that won’t carry through to the iMac.
Can you help me understand how this (or your other Thunderbolt dock) works so I can determine if the USB3 ports will work?
We have had reports that our TBT3-UD1-83 dock will work when connected to an Apple system with Thunderbolt 2 ports in conjunction with the Apple Thunderbolt 3 to 2 adapter, but this not something we officially support or test in our lab.
We have not had any reports from customers using the Apple adapter with a system that only has Thunderbolt 1 ports, so we cannot speak definitively as to what will or will not work in that type of setup (nor would such a configuration be officially supported). To your question, there is a Fresco USB 3.0 host controller built-in to the TBT3-UD1-83 dock itself.
Should you elect to proceed with a purchase and find that the dock does not work as expected in the configuration you describe, we do offer a no-hassle 30-day return policy for all products sold by Plugable and fulfilled by Amazon. Conversely if things do work as expected we would appreciate the feedback as it will help us to help others in the future.