Is this install downloaded from "My products" in our page -> account (or login) at the top right?
You should have got it from here:
It should say "Registered version":
And then it should show Delphi 11.3 (if 11.3 is correctly registered in the registry).
If you have the registered setup and 11.3 is not showing, you can try running
setup /showall=true from the command line (As explained in the page where you select the version):
But if you need to do the showall=true, there is something wrong with the 10.3 install, it might worth reviewing it.