Joint invariants of a pair of coplanar conics have been largely studied for object recognition. In this paper, we are concerned with the invariants of a triplet of coplanar conics. It will be shown that, besides of the invariants associated to all pairs of three conics, there exist only one invariant which is associated to the triplet of conics. All joint invariants of a triplet of coplanar conics will first be derived based on the invariant algebra of a net of ternary quadratic forms. Then, experimentations for discriminating power, accuracy and stability of joint invariants are conducted both for simulated and real images.