The latest version of LEGO Digital Designer (4.3.12), as available from the LEGO website or other sources, does not contain any of the EV3 parts and may contain other omissions. The prior version (4.3.11) is the most feature complete, but does not install well onto Windows 10. Here is how to install it anyways.
Note, 4.3.12 calls itself 4.3.11 after it is installed, so even if your version says it's 4.3.11 it may not be.
Unfortunately this file is no longer available from LEGO directly, but it can be retrieved courtesy of the internet archive project. The file is ~312MB large and can be dowloaded at the following link:
Once the installer has finished downloading, start it but do not begin installation. Stop when you get to the screen that says "Welcome to the LEGO Digital Designer Setup Wizard":
Press Windows-R to open the Run dialog and paste then run the following command:
CMD /c "DEL %TEMP%\WZSE0.TMP\install_flash_player_active_x.exe && COPY %TEMP%\WZSE0.TMP\OpenGLChecker.exe %TEMP%\WZSE0.TMP\install_flash_player_active_x.exe"
Then wait for a black window to appear briefly then disappear. The contents of the window don't matter, don't worry if you aren't able to read them before it goes away.
Continue stepping through the installer. It will tell you your adobe is out of date. Click "Yes" and keep going. You're almost there!
Congratulations!