collectionView scrollToItemAtIndexPath

- (void)scrollToItemAtIndexPath:(NSIndexPath *)indexPath atScrollPosition:(UICollectionViewScrollPosition)scrollPosition animated:(BOOL)animated;

滚动的item 数量由 indexPath的 item来决定 如果你的页面是三个商品一个页 

那么滚动到下一页 应该是 indexPath.item = currenPage * 3