In this work, we present a new class of tristable mechanism called double Young tristable mechanisms (DYTMs), which connect two prestrained Young bistable mechanisms to create three distinct stable equilibrium positions. A three-degree-of-freedom pseudorigid-body (RPB) model is proposed to accurately predict the kinetostatic behaviors of both Young mechanisms and DYTMs. An optimization-based design method is also presented for DYTMs. Two DYTM prototypes were designed based on the method and machined out of polypropylene sheets. Both of the prototypes exhibit tristability, which demonstrate the feasibility of achieving tristability through connecting two prestrained Young mechanisms. The successful prototyping also indicates that the proposed three degree-of-freedom (3DOF) model is capable of identifying feasible designs for DYTMs.