Sublime Text 3 highlights all instances of currently selected text. Is there any shortcut to navigate cursor to the next / previous instance (copy) of selected text?
I may be misunderstanding your question but what you are asking seems pretty simple and I wonder if that means you might mean something else. Anyway here's an answer that I hope is what you are looking for.
First, ensure that you have the following set to true in your preferences:
// If true, the selected text will be copied into the find panel when it's
// shown.
// On OS X, this value is overridden in the platform specific settings, so
// you'll need to place this line in your user settings to override it.
"find_selected_text": true,
Select the string you are interested in.
Search in current file [OS X Cmd+F :: Windows Ctrl+F]
Now you can step through the matches:
Some more here :: http://www.cheatography.com/njovin/cheat-sheets/sublime-text-2-keyboard-shortcuts-windows/
What am I missing in your question?