I think this is a better way of centering columns when using flexbox and borders between them. nth child the 1st item and the last item with no padding. This also repeats down the line if more than 3 columns are used

three-column.scss

.multi-column {
  .multi-column__col {
    flex-basis: 33%;
    margin: 0;
    padding-left: 20px;
    padding-right: 20px;
    &:nth-child(3n+1) {
      padding-left: 0;
    }
    &:nth-child(3n+3) {
      padding-right: 0;
    }
  }
}