It is more reasonable to assume this than to imagine, with Mr. French, that these three formed the entire British episcopate. And there is reason to suppose that York, London, and Caerleon were metropolitan sees.
Canon x.: De his qui conjuges suas in adulterio deprehendunt, et iidem sunt fideles, et prohibentur nubere; Placuit ... ne viventibus uxoribus suis, licet adulteris, alias accipiant. [Haddan, 'Concilia,' p. 7.]
'Ad Jovian' (A.D. 363).
'Contra Judaeos' (A.D. 387).