Hello everybody. And thank you MaLd0n.
Basically, I was able to upgrade and install Big Sur Beta 2 to Beta 3 by the method of this Web page.
Use the latest build of OpenCore and Kext.
What I did not notice was to select preboot at boot time after the upgrade.
Details are on my web page.
With BigSurDevBeta3 upgrade installation (USB memory), I was able to boot.(include bootup movie)