From The Collaborative International Dictionary of English v.0.48:
Tri- \Tri-\ [Gr. tri- or L. tri-, sometimes through French; akin
to L. tres three, and E. three. See Three.]
[1913 Webster]
1. A prefix meaning three, thrice, threefold; as in
tricolored, tridentate.
[1913 Webster]
2. (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.
[1913 Webster]