I suggest you try to get the three ADCs working first. Use 3 DACs which will accept the codes got from the ADC. This way you would be able to check by turning the potentiometers, and looking into the DAC outputs. 3 DAC6 UMs should be fine for the three ADCs.
Also, using a timer to time the sinusoidal samples would be better, otherwise the sinusoid would be jittery.
125Khz sample rate is too high to be able to compute sine.Have you considered using a look-up table instead?