Using the 3 Ka's separately with the formula:
x^2 = 8.4x10^-4 x 4.2x10-1
x = 1.87x10^-2 moles / L
Then x^2 = 1.8x10^-5 x 4.2x10-1 moles / L
x ==2.7x10^-3
the last one, x = 1.3x10-3 moles / L
Add them with the decimal points in line , and the indices all x10-2 we get, 2.27x10^-2
pH = 1.65