When robots interact with the environment, it is necessary for them to detect and control contact forces. In many cases, force/torque sensors or tactile sensors on robots do not contact the environment directly, but instead contact through end-effectors or grasped objects. Thus, robots have to estimate actual contact forces through insensitive objects. However, estimating multiple contact forces through insensitive objects is an ill-posed problem. This paper proposes a method of estimating contact forces on three points separately with a six-axis force/torque sensor on an end-effector. We discovered that the components of contact forces normal to the plane defined by the three contact points can be found uniquely by applying static equilibrium of force and moment. The tangential components of contact forces can also be determined by the use of an additional algorithm. The validity of the proposed method is confirmed by experimental results.