Angular Flex-layout with ngFor, X elements per row

Question

I tried to follow this post Angular flex-layout with ngFor to do what I want but I can't make it.

I have this code:

<div *ngFor="let room of listRoom;">
  <div class="room">
    Room {{room.label}}
    <div *ngFor="let bed of room.beds;">
      <div class="bed">
        Bed #{{bed.label}}
        <button mat-raised-button color="accent" (click)="assignThisBed(bed)">
          Assign this bed
        </button>
      </div>
    </div>
  </div>
</div>

My listRooms contains usually between 10-15 elements and I'd like to have 3 rooms on a row then breakline, 3 rooms, ...

Can someone help me? Thanks..

Here is a StackBlitz with my code.