Angular does not provide this functionality out of the box. I think that the simplest way to achieve the desired result is to only display data on every third index like so:

<div class="list-group">
  <div *ngFor="let p of products; let idx = index" >
    <div class="row" *ngIf="idx % 3 === 0">
      <app-product [product]="products[idx]"></app-product>
      <app-product [product]="products[idx+1]"></app-product>
      <app-product [product]="products[idx+2]"></app-product>
    </div>
  </div>
</div>

Demo