Digi Showcase is responsive friendly.
It's based on Bootstrap 2.3.2. So please ensure that this default Joomla 3.x framework is loaded into your pages. It may be a template issue; your template may use another framework, disabling the default Bootstrap.
If Bootstrap is loaded correctly, so please be more precise about the issue. Attach a screenshot or an URL where is possible to see that issue.
I've checked your screenshot.
Helix Ultimate uses Bootstrap 4, so probably the issue is caused by that. The module works on Bootstrap 2.3.2 as it's the Joomla 3.x default scaffholding framework.
There is nothing else that can be done, the only possibility is to make an override of the module view and manually change the column CSS class from "spanX" to "col-X" or "col-lg-X col-sm-X col-xs-12".
The module will be updated with Bootstrap 4 in the future, of course, but as not Joomla 3 uses the version 2.3.2, we are keeping to use that one.
Wait, if you are using Carousel mode there is nothing wrong. Carousel has not responsive columns. Its based on Slick Carousel. I mean that if you set 3 slides, you will have 3 slides in all monitor resolutions. You can set only 1 slider or try the vertical slider mode.