I have found the answer to your question I have reproduced it here:
https://jsfiddle.net/nmcobalt/dg3mutwv/13/
In order to do the css less complicate and solve this complex issue, it is recommended to use SCSS* instead of plain css.
In SCSS file you have to use the css selector nth:last-child with iteration calculating childs based on modulo function and the constant factor of the 3 columns.
@mixin setLastChildFluid{
  @for $i from 1 through 10 {    
    $span: 3;
    @if $i % 3 == 0{
      $span: 1;
    } @else if $i % 3 == 1{
      $span: 3;
    } @else if $i % 3 == 2{
      $span: 2;
    } 
    @if $span == 2 {
      span:nth-child(#{$i - 1}){
        &:nth-last-child(2){
          grid-column-end:span 1;
          position:relative;
          bottom:0;
          width:calc(150% + 5px);
          left:0;    
        }
      }
      span:nth-child(#{$i}){
        &:nth-last-child(1){
          grid-column-start:2;
          position:relative;
          bottom:0;
          width:calc(150% + 5px);
          left:calc(50% + 5px);
        }
      }
      
    } @else {
      span:nth-child(#{$i}){
        &:last-child{
            grid-column-end:span #{$span};          
        }
      }
    }
    
  }
}
div {
  position:relative;
  margin:20px auto;
  width: 400px;
  background: #d8d8d8;
  display: grid;
  grid-gap: 10px;
  grid-template-columns: repeat(3, 1fr);
  
  @include setLastChildFluid;
}
span {
  height: 50px;
  background: blue;
  color:yellow;
}
and the following html markup; I have done two different examples (wrapped by div) to reproduce your query:
<div>
  <span>1</span>
  <span>2</span>
  <span>3</span>
  <span>4</span>
  <span>5</span>
  <span>6</span>
  <span>7</span>
  <span>8</span>
</div>
<div>
  <span>1</span>
  <span>2</span>
  <span>3</span>
  <span>4</span>
  <span>5</span>
  <span>6</span>
  <span>7</span>
</div>
Change the number of divs in order to see the fluid result.
*SASS is a preprocessor scripting language that is interpreted or compiled into Cascading Style Sheets (CSS). Learn more here: https://sass-lang.com/guide