<h3 class="title">
  <span class="main">Main text main main main main</span>
  <span class="sub">sub sub usb sub sub sub sub sub sub sub</span>
</h3>

.title {
  width:50%;
  text-align: justify;
  line-height:0;
  
  .main,.sub {
    display: inline-block;
    line-height:100%;
  }
  
  .sub {
    color: grey;
  }
  &:after {
   content: "";
   display: inline-block;
   width: 100%;
   border-top: 1px solid grey;
  }
}