Can I link a .blend file instead of separate object? If I link objects separate, I have to link all new objects one by one if I change the original file, isn't it?

if I want to link a file which has 3 objects, I have to link 3 objects separately, I can't just link the .blend file, isn't it? But if I add another 5 objects in the external .blend file, I have to link those 5 objects one by one into the .blend I'm working in, they won't be added in the .blend I'm working in automatically, because I didn't link them before. Is there any way to let them be added automatically?
No, as their name tells you: links are... A link between two files. You make a link between something in an external .blend and the .blend you're working in.
No matter what you have linked, it will update once the changes are saved in the linked file and the link reloaded (or the entire working blend reloaded).