Showing full posts using a single.php template

i have a category of three posts showing in a row across a homepage.
they consist of a thumbnail image, a <h5> tag and an excerpt.

when i click the permalink of any one of these posts, i would like the action to bring me to the ‘full post’ …showing the full image and the content, but i dont know how to do this. I was told to use the single.php file but, if there was a number of categories they’d all be the same but i want to have the option of whether i want to show a full image etc on one category and not the next.

would be great if someone could show me how to ‘properly’ use the single.php page, what code to insert and how to attach it to a category…

As default of WordPress,
single.php file always show full content of post.
you have to do nothing.