If you put 3 testimonials next to each other in different columns and they have a different amount of text, then they all have a different height.

@media (min-width: 981px){
.home .et_pb_column_1_3 .et_pb_testimonial {
min-height: 284px;
}
}
Code language: CSS (css)

Other option is to set an equal column height on the row's advanced settings and set a background colour for each column there rather than in the module.