Provide details on installation; if Home Assistant OS or not, exactly on what hardware/platform, etc.?
If you not using Home Assistant OS directly on bare-metal hardware then you might first have to jump through some hoops to install CP210x (USB to UART Bridge VCP) Drivers and configure correct settings for USB pass-through if using a virtual machine, etc. (and if you are using a container-based installation then need to configure USB device forwarding to the Docker image or similar).
Before starting also first upgrade the firmware on the ConBee 3 manually using a different computer to confirm that the adapter is working at all. If you manage to upgrade the firmware then at least you know that it should be possible to connect to it → https://phoscon.de/en/conbee3/install#hazha
Regardless of setup be sure to always use a long USB extension cable with good sheilding (either a USB 2.0 extension cable to any USB port or a USB 3.0 extension cable to a USB 2.0 port) to get it away from EMF sources, see → Zigbee networks: how to guide for avoiding interference + optimizing using Zigbee Router devices (repeaters/extenders) to get best possible range and coverage