  Using the helicity-spinor language we explore the non-perturbative
constraints that Lorentz symmetry imposes on three-point amplitudes where the
asymptotic states can be massive. As it is well known, in the case of only
massless states the three-point amplitude is fixed up to a coupling constant by
these constraints plus some physical requirements. We find that a similar
statement can be made when some of the particles have mass. We derive the
generic functional form of the three-point amplitude by virtue of Lorentz
symmetry, which displays several functional structures accompanied by arbitrary
constants. These constants can be related to the coupling constants of the
theory, but in an unambiguous fashion only in the case of one massive particle.
Constraints on these constants are obtained by imposing that in the UV limit
the massive amplitude matches the massless one. In particular, there is a
certain Lorentz frame, which corresponds to projecting all the massive momenta
along the same null momentum, where the three-point massive amplitude is fully
fixed, and has a universal form.
