  An operator T on Hilbert space is a 3-isometry if there exists operators B
and D such that (T*)^n T^n = I+nB +n^2 D. An operator J is a Jordan operator if
it the sum of a unitary U and nilpotent N of order two which commute. If T is a
3-isometry and c>0, then I-c^{-2} D + sB + s^2D is positive semidefinite for
all real s if and only if T is the restriction to an invariant subspace of a
Jordan operator J=U+N with the norm of N at most c. As a corollary, an
analogous result for 3-symmetric operators, due to Helton and Agler, is
recovered.
