There are two ways to get the same totals. Each approach has its advantages.
Define summary fields in Items that compute the totals of each of the three fields. When you place the summary field from this table on your assortment layout, it will total the data from the related records only.
Define three calculation fields in your assortments table. Use Sum ( RelatedITems::IsNew ) type syntax to compute totals of your sets of related records.