The concentration of H+ ions can be calculated from the pH values:
[H+]=10−pH
For the first solution (pH = 3): [H+]1=10−3 M
For the second solution (pH = 4): [H+]2=10−4 M
For the third solution (pH = 5): [H+]3=10−5 M
Let the volume of each solution be V.
Total moles of H+ in the mixture = (10−3+10−4+10−5)×V=(100×10−5+10×10−5+1×10−5)×V=111×10−5×V=1.11×10−3×V.
Total volume of the mixture = V+V+V=3V.
The final H+ ion concentration = Total volumeTotal moles of H+=3V1.11×10−3×V=0.37×10−3 M=3.7×10−4 M.