The Collaborative International Dictionary of English v.0.48:

Trinal \Tri"nal\, a. [L. trinus triple, trini three each, fr.
   tres, tria, three. See Three, and cf. Trine, a. & n.,
   Tern, a.]
   Threefold. "Trinal unity." --Milton.
   [1913 Webster]

         In their trinal triplicities on high.    --Spenser.
   [1913 Webster]