I've just begun a basic HTML page to get to grips with implementing Fancybox-3 into my sites code. When I select either of the two thumbnails, their full size IMG appears (as expected), but, another 'C:/' appears in the URL! This is compounded each time I select the 'Next' or 'Back' arrow on either image, as this inserts yet another, 'C:/' into the ever expanding URL.
e.g. file:///C:/C:/C:/C:/Users/Peter%20Johnstone/Desktop/...
It looks to me the issue lies in the fact that your are probably running your local website with localhost.
The solution is to create a virtual host on your machine so that you can have a functional web root.
You don't mention which technology you are using, in case it's Apache, have a look on one way to do it here.