Let \(T_{r}\) be the \(r^{\text{th}\text{~}}\) term of an A.P. for \(r = 1,2,3\), \(\_\_\_\_\) If for some positive integers \(m,n\) we have \(T_{m}=\frac{1}{n}\) and \(T_{n}=\frac{1}{m}\), then \(T_{mn}\) equals