I only have one USB-C on my iMac.
Is there a USB-C Hub that will give me at least two more ?
I Haven't found any.
One USB-C goes to the HUB and the HUB has at least two better if 3 USB-C that I can use.
One is used by the external monitor. The other with external HD.
There shouldn't be a speed issue or am I mistaken?
Neither of the above, nor any other device at this time provides more than just 1 usb-c port. Looking around forums reveals much information about this (whether it's true or not I cannot confirm). Many sites indicate that the chip required for USB-C distribution currently does not exist and some sites indicate this will be possible some time in 2020. Use your own judgement as to the credibility of these sites and forums - but I also haven't been able to locate any dock that provides multiple USB-C ports.
Besides this there have always been chronic issues with USB hubs not providing the full voltage/current capability of the native Mac ports. This is regardless of whether the hubs are powered or not. There are many USB "power negotiation" protocols between source and sink.
If the 3rd-party hub chipset does not properly support Apple's protocol, you will encounter cases where a spinning bus-powered USB drive will go off line, or maybe two plugged into the same USB-A hub will go off line, whereas of those drives were plugged into the Mac ports they work fine.
There are relatively inexpensive yet sophisticated test devices which allow examining this, while placing an artificial variable load on the USB port.