This was a conflict from your theme's
grid-base.css.
Our plugin uses bootstrap for the design and your theme also seems to be using bootstrap. We use the bootstrap's
col-md-4 class when you set the column count to
3. The standard bootstrap library will reset the size of the column element to full width in small devices. But, your theme overrides the same class and always sets the width of the element to 33%. So, this issue was created by your theme's style-sheet.
Kindly try using the following CSS code,
@media (max-width: 768px) {
.col-md-4 {
width: 100% !important;
}
}