This 21.02.3 will not include support to the Archer C6 v3.
The next version that will start including support to Archer C6 v3 is 22.03. You can download the snapshot builds of 22.03 here. Notice that albeit being a snapshot image and therefore not a stable channel, the 22.03 snapshot builds includes LuCI by default and they are is more stable than the master snapshots.
Anyway please see below about using unreleased stable versions (such as 21.02.3 and 19.07.10):
https://forum.openwrt.org/t/a-note-about-using-new-stable-releases-before-they-are-announced