The Roman Map of Britain Toesobis, Gois-orios or Trisorios ? Tremadoc Bay

ToisobioV potamou ekbolai Toesobis fluvii ostia, var.Tisobiou Tisobis (Ptolemy II 3 2) 

Tois- for Gois- and obioV for orioV 'wide (ill-defined?) boundary/landmark'? The obscure Greek word goisoV is found in the Etymologicon Magnum 238.45 : melan 'dark', platu 'broad, wide'. Was Ptolemy transliterating a name or translating?

Tremadoc Bay seems dictated by the position on the map, likely Afon Dwyryd is the river mouth in question. PNRB suggests Afon Dwyryd or Afon Glaslyn.

    An error of Greek 'o' for 'r' would give a first element Tris- 'three, thrice' regularly used as an intensifier. Tris-orioV 'three landmarks' river-mouth?
    Perhaps it should be divided as Tri-sorioV with the second element as ERN Soar p. 374 with Soar Brook, two Sor Brooks, and Sorgwm. CDEPN has Soar derived from PrW *Sâr as in Sarua and Saravus.