You'll need to log in to the Broadcom Support Portal with an entitled account to download the ESXi 8.0U3b installation bundle. Since the transition from VMware to Broadcom, free accounts no longer have access to installation media only customers with active support contracts can download it.
Here's the official link:
https://support.broadcom.com/group/ecx/downloads
If you don't have entitlement, you'll need to go through a Broadcom partner or your hardware vendor for access.