Who so Pullith out this Sword from this Stone is the true born King of all Britton. 

The Three Rings represent Unity.

Excalibur, King Arthur's magical sword symbolizes both destruction and fertility. In one version of the legends of Arthur, the future king proved his right to rule by pulling Excalibur out of a stone, which no other could do. In another version, he received the sword from the Lady of the Lake. After his battle with Mordred, as Arthur lay dying he asked Sir Bedevere, one of his knights, to return Excalibur to the lake, where an arm rose up out of the water to receive it.

The Sword in the Stone, was drawn by Arthur  as proof of his birthright to be King and of his nobility. It is both a test and a miraculous sign of his Royalty. The sword drawn from the stone is different from the one given to Arthur by the Lady of the Lake. The latter is always referred to as Excalibur; the former is called by that name only once, when Arthur draws the sword at a crucial moment in the first battle to test his sovereignty.