There's a copy command all right. You can copy stuff in your own map or copy it to the clipboard and paste in a new file.
I don't have CC3 installed on this computer, but it's an icon on the right. It's a button called "Move" or "Copy". You can right click on it to get options like copying to clipboard. Select the entities you want copied and right click and select "Do it!" If you copied to clipboard you can then open a new file and select paste from the menu on the top of the screen.