Hello,

Try to add the following code snippet to the Custom CSS area in Theme Settings to fix this issue.

@media (max-width: 991px){
   .testimonials.testimonials-grid{
        flex-direction: row;
        -ms-flex-direction: row;
    }
    .testimonials.testimonials-grid.testimon-columns-3 .testimonial{
        -webkit-flex-basis: 100%;
        -ms-flex-basis: 100%;
        flex-basis: 100%;
    
    } 
}