St. Leo's own words:
And so under the first head is shown what unholy views they hold about the Divine Trinity: they affirm that the person of the Father, the Son, and the Holy Ghost is one and the same, as if the same God were named now Father, now Son, and now Holy Ghost: and as if He who begot were not one, He who was begotten, another, and He who proceeded from both, yet another; but an undivided unity must be understood, spoken of under three names, indeed, but not consisting of three persons. This species of blasphemy they borrowed from Sabellius, whose followers were rightly called Patripassians also: because if the Son is identical with the Father, the Son’s cross is the Father’s passion: and the Father took on Himself all that the Son took in the form of a slave, and in obedience to the Father. Which without doubt is contrary to the catholic faith, which acknowledges the Trinity of the Godhead to be of one essence in such a way that it believes the Father, the Son, and the Holy Ghost indivisible without confusion, eternal without time, equal without difference: because it is not the same person but the same essence which fills the Unity in Trinity. (Letter 15, Quam laudabiliter, To Turribius, Bishop of Asturia, 447 AD)
I'll see about getting back to you on St. Leo III.
Please notice the tenses here in what you quoted: "... as if He who begot were not one, He who was begotten, another, and He who proceeded from both, yet another."
This is not, as is the filioque heresy, a discussion of the eternal hypostatic procession of the Holy Spirit. It is instead a reference, at least apparently, to there being three distinct persons and to the Son's mission in time. For example, Holy Scripture tells us, "And when he had said this, he breathed on [them], and saith unto them, Receive ye the Holy Ghost" (Jhn 20:22). That, it appears to me, is what St. Leo is referring to. The Creed, by contrast, refers to the hypostatic eternal generation of the Son and to the hypostatic eternal procesion of the Holy Spirit, an entirely different subject. Of this, Jesus Christ Himself said, "But when the Comforter is come, whom I will send unto you from the Father, [even] the Spirit of truth, which proceedeth from the Father (Jhn 15:26 )".