The Collaborative International Dictionary
Tri- \Tri-\ [Gr. tri- or L. tri-, sometimes through French; akin to L. tres three, and E. three. See Three.]
A prefix meaning three, thrice, threefold; as in tricolored, tridentate.
(Chem.) A prefix (also used adjectively) denoting three proportional or combining part, or the third degree of that to the name of which it is prefixed; as in trisulphide, trioxide, trichloride.
Etymology 1 pre. three Etymology 2
pre. (context chemistry English) Used in combination with a known element name (e.g. 'tri-silicon') to designate an unknown element that would reside three periods lower than the known element on a Mendeleev-type periodic table. See http://en.wikipedia.org/wiki/Mendeleev's%20predicted%20elements.
Douglas Harper's Etymology Dictionary
word-forming element meaning "three, having three, once every three," from Latin tres (neuter tria) or Greek treis, trias "three" (see three).