The crest parts are items obtained during the Family Crest quest. Each part is owned by one of three Fitzharmon brothers, who have all left their family home. Each brother took a separate piece of the crest with him to prevent the other two brothers from inheriting the family fortune.
The player must complete a set of tasks for each of the brothers as follows:
Once the player has all three crest pieces, they can be combined to make the whole family crest, which can then be given back to Dimintheis Fitzharmon, the boys' father, in Varrock.
For an exhaustive list of all known sources for this item, see here