Category Switches Back and Forth

Help! I’m not a coder, but I’ve managed to get myself this far: I have three categories I want to include on my site. I’ve successfully created different templates for the three categories, so that if you click the category name, you see the new template and the list of entries and excerpts for that category.

HOWEVER, when you click on an individual post to view the entire post (and not just the excerpt) it displays it in the main (default) category template, NOT the template I’ve created for that particular category entry.

Can anyone help?