Using an IDE might help. I use NetBeans. When I have a .jar for an add-on to java I must start a Project, then Right-click the project name to get a context menu for Properties. That brings up a dialog box to include .jars into the project. BTW, when you include a .jar that get copied into the final program distribution. A folder will not copy over. Okay, what that does is include the .jar in the classpath for every time I compile and / or run my code.
Now that I have Java3D (or whatever) I can type in my code window such as you have:
SimpleUniverse uni - new SimpleUniverse();
that will cause NetBeans to discover that I need the import statement at the top of the code list. BTW, I noticed you don't have the SimpleUniverse(); The parenthesis is the call to the constructor of SimpleUniverse.
I don't have the Java3D API in front of me, but the import would look something like...
import com.java3d.SimpleUniverse;
I do have a very large folder of all kinds of .jar files. My folder is named Java_libraries. An IDE is what began to show me the importance of 'import', 'private' and all the other linking modifiers.
Lastly, you might also investigate JMonkeyEngine for 3D.