Here is another example of "these three are one" with no reference at all to water, spirit and blood.
Caesar de Missy
Against Praxeas XXV
“So the close series of the Father in the Son and the Son in the Paraclete makes three who cohere, the one attached to the other: And these three are one substance, not one person, in the sense in which it was said, ‘I and the Father are one’ in respect of unity of substance, not of singularity of number."
"Thus the connection of the Father in the Son, and of the Son in the Paraclete, produces three coherent Persons, who are yet distinct One from Another. These three are one [thing], not one [Person], as it is said, 'I and my Father are One,' in respect of unity of substance not singularity of number." (ANF vol 3, p. 621) http://www.tertullian.org/works/adversus_praxean.htm
Ita connexus Patris in Filio, et Filii in Paracleto, tres efficit cohaerentes, alterum ex altero, qui tres unum sint, non unus. Quo modo dictum est (Ioan. X, 30): Ego et Pater unum sumus; ad substantiae unitatem, non ad numeri singularitatem. (Migne Latina, PL 2, 0187D)
despite being an ultra-contra, might actually give a better English translation,
“He saith, lie shall take of Mine (John 16:14), even as He Himself of the Father. Thus the connexion of the Father in the Son, and of the Son in the Paraclete, maketh Three that cohere together one from the other: which Three are one Substance, not one Person; as it is said, 7 and My Father are one (John x. 30), in respect to unity of essence, not to singularity of number.”
(Adv. Praxean xxv.).
Thus the connexion of the Father in the Son, and of the Son in the Paraclete, makes three coherent ones from one another, which three are one (unum, not unus,i.e. one substance, not one person) as it is said, “I and my Father are one,” denoting the unity of substance, not the singularity of number. -
(Samuel Davidson, Lectures on Biblical Criticism, p. 135, 1835)