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I think there is not enough data to provide an exact answer theoretically. You can figure out an estimate value by virtually titrating the tripeptide and figuring out after what step an isolectric point likely situates: $$\def\VEQ{{\scriptsize-\ce{H+}}{\Large\downharpoonleft\!\!\upharpoonright}\scriptsize{+\ce{H+}}} \begin{array}{rcc} \mathrm{pH} < 2.2 &... 1 From here:$$\mathrm{pH} = \frac{1}{2}(\mathrm{p}K_\mathrm{a} + \mathrm{p}K_\mathrm{w} - \mathrm{p}K_\mathrm{b}) Plugging in $\mathrm{p}K_\mathrm{a} = 7.04$ and $\mathrm{p}K_\mathrm{b} = 4.74$, you get $\mathrm{pH} = 8.15$. As mentioned by Poutnik and Mithoron in the comments, $\ce{HS-}$ can be neglected as an acid due to a low $K_\mathrm{a2}$ value. So ...