From Platinum onwards, the baby will always inherit 3 random IVs from the parents (For example, it will have the same IVs for HP and Attack from the father, and the same IVs for Speed from the mother). The other 3 IVs will be random.
When a parent holds the Destiny Knot, the baby will inherit 5 IVs instead of 3 (For example, HP, Attack and Defense from the father, and Sp.Atk and Speed from the mother.) In that case, only 1 IV would be random (Sp. Def).