Summary: We consider a quantum mechanical three-particle system made of two identical fermions of mass one and a different particle of mass \(m\), where each fermion interacts via a zero-range force with the different particle. In particular we study the unitary regime, i.e., the case of infinite two-body scattering length. The Hamiltonians describing the system are, by definition, self-adjoint extensions of the free Hamiltonian restricted on smooth functions vanishing at the two-body coincidence planes, i.e., where the positions of two interacting particles coincide. It is known that for \(m\) larger than a critical value \(m^\ast\simeq(13.607)^{-1}\) a self-adjoint and lower bounded Hamiltonian \(H_0\) can be constructed, whose domain is characterized in terms of the standard point-interaction boundary condition at each coincidence plane. Here we prove that for \(m\in (m^\ast,m^{\ast\ast})\), where \(m^{\ast\ast}\simeq (8.62)^{-1}\), there is a further family of self-adjoint and lower bounded Hamiltonians \(H_{0,\beta}\), \(\beta\in\mathbb R\), describing the system. Using a quadratic form method, we give a rigorous construction of such Hamiltonians and we show that the elements of their domains satisfy a further boundary condition, characterizing the singular behavior when the positions of all the three particles coincide.