\textbf{(*)} 0=\lim_{h=(h_1,h_2,h_3) \rightarrow 0}\frac{f(T_0+h)-f(T_0)-Df(T_0)h}{\|h\|}=\lim_{h=(h_1,h_2,h_3) \rightarrow 0}\frac{f(T_0+h)+h_1}{\|h\|}