Make a post/Category show only excerpt

Hi!

Im trying to find out how to make posts belonging to a certain category to show just excerpt, but doesn’t have any luck. PHP is new to me so don’t really get it.

Here is the link to my site: https://www.svenssonsbild.se/Magasin
As you see, theres a post i have that is made for wide viewing, and it works great everywhere except on featured posts, and i would like to have it show less there.

I tried to go into featured.php, and i was thinking something like this should do it:

<?php if ( is_category(3) ) { ?>
<?php the_excerpt(); ?>

<?php } else { ?>
<?php the_content(); ?>

<?php } ?>

Haven’t had any luck so far, any pointers?

Regards,
Jay