  We use the representation theory of the quasisplit form G of SU(3) over a
p-adic field to investigate whether certain quotients of the Bruhat--Tits tree
associated to this form are Ramanujan bigraphs. We show that a quotient of the
tree associated with G (which is a biregular bigraph) is Ramanujan if and only
if G satisfies a Ramanujan type conjecture. This result is analogous to the
seminal case of PGL(2) considered by Lubotzky-Phillips-Sarnak. As a
consequence, the classification by Rogawski of the automorphic spectrum of U(3)
implies the existence of certain infinite families of Ramanujan bigraphs.
