A particle is constrained to move along the \(x\)-axis in a region where its potential energy \(U(x)\) is given by the smooth function shown in the graph. The curve has a local minimum at position \(x_1\), a local maximum at position \(x_2\), and an extended horizontal plateau of zero slope and zero curvature at position \(x_3\). Which of the following correctly classifies the type of equilibrium at positions \(x_1\), \(x_2\), and \(x_3\)?