tri·va·lent 
(trī-vālənt)
(trī-vālənt)adj.
1. Chemistry Having valence 3.
2. Immunology
a. Having three sites of attachment. Used of an antibody or antigen.
b. Containing antigens from three strains of a microorganism or virus. Used of a vaccine or serum.
tri·valence, tri·valen·cy n.