Get the frequency of a three phase voltage without a PLL?
What is it exactly that you are trying to do? Do you mean three phase voltage as in the power grid? If you need to get the frequency of a three phase voltage of the power grid, there are basically two options. 

First is to do what you call zero crossing detection: you calculate the number of zero crossings per time unit. That is an easy way to get the frequency, but it takes a relatively long time. For example 1 second to get an accuracy of one hertz.

The second option is to get a moderately long vector of samples of the voltage. Something like 5..10 waves or more. Then Hann-filter the vector and run a correlator to it. You know, multiply it with 0.01 Hz sine and cosine signals and calculate the power; then repeat for 0.02 Hz, then 0.03 Hz and so forth. Of course you probably already have a fairly good estimate of the signal's frequency; if you know it's about 60 Hz, then you can start the correlator at 59.00 Hz and step from there. 

As a side note: someone might suggest you to do a fast Fourier transform (FFT), but what is FFT anyway? It's a series of correlator banks. If you already roughly know the frequency, there's no need to calculate the whole frequency spectrum from zero to Fs/2. Just correlate the range of frequencies that you need based on the rough estimate.

The power grid is noisy, so any immediate samples you get are too noisy to detect any phase information with any accuracy. You need longer sample periods and/or filtering.

Phase has no relevance to your titled question. But you can calculate different kinds of phase information from the voltage samples; you pick one of the voltages as the phase reference and compare others to it. And if it's energy meters you're involved with, doing the same with currents also opens up the possibilities of calculating active, reactive and apparent powers and energies.