Please note that ADB Sideload doesn’t work with the image files currently offered online with the manual install instructions for /e/OS on FP3 (because the installation currently relies on Fastboot instead of ADB).
However, links to install files in the OTA format suitable for ADB Sideload are available via https://ota.ecloud.global/api/v1/FP3/dev (see the “url:” lines there).