Since you are changing something on your frontend, you have to temporary turn off your site cache like :
- Joomla global configuration cache system
- System - page cache plugin
- JCH - optimise plugin
- After turn off all these, you have to clear the cache from your site backend
If not, it will not get any effect when you changing the setting from backend, because your page HTML content already get cached on the site.
Now you can see your showcase in the Grid layout now, since your current site only 3 blog post, so it doesn't show other post in the grid layout because that showcase already show all the 3 post.
For now, i already help you re-enabled back those cache setting what i described at above.