PLL and zero crossing detection
What about this: if your three signals are balanced (I mean with equal amplitudes and $2\pi/3$ shifts), you can find the amplitude by summing the squares (check that it doesn't pulse). And from there, the arguments of the three sinusoids ($\omega t+\phi+k2\pi/3$). Actually you solve this geometric problem, where the three projections are given:



Then, repeating the computation at two close instants, you get the frequency. It may be useful to maintain a linear fit on the last few $n$ phase values to improve accuracy and ease handling of phase wraparound.