Машинопись
imageGroups = this.yourImageArray.reduce((p, c, i) => {
if (i === 0 || i % 3 === 0) {
p.push([c]);
} else {
p[p.length - 1].push(c);
}
return p;
}, []);
HTML
<carousel>
<slide *ngFor="let group of imageGroups">
<img *ngFor="let image of group" [src]="'https://' + image.src" [alt]="image.description" style="display: inline-block; width: 30%;" />
</slide>
</carousel>
Демо здесь