you say the 18f2431 is specifically for frequency measurements
The PIC is not specifically for measuring frequencies. It just has a capture module
that makes it much easier to do since it will capture the value of Timer5 on every
rising edge - which is the period of the square wave.

also you say it does two simultaneously?
It can take 2 A/D readings simultaneously.

i have 3 phases from my analogue signal, which are split into 3 different signals, all of which i need to measure the frequency of and get an average. is this still possible with this pic?
Yes.

and would using the 18f2431 be alot easier than using a schmitt trigger and just measuring the frequency of 3 square waves using interupts? would there be 3 places on the pic i could make these inputs aswell?
Yes it has 3 capture inputs, and using built-in hardware to measure frequency is a good
deal easier than interrupts. All 3 capture inputs just happen to be Schmitt Triggers inputs
too.

If you do need to use interrupts, then I definitely recommend you take a look at Darrel
Taylors' Instant Interrupt routines. He's done some awesome work on these.