FHSS-2 is a little slower response time. 99% of people wont tell the difference at all. the FHSS-3 is the fastest receivers airtronics offers. most i see get the FHSS-2 for spares or for their other vehicles. either one works just fine.
if you get a FHSS-2 for your MX-3x, all you do is bind and go. very simple. make sure its FHSS-2 or FHSS-3, no others work... there are DSSS (i think thats right, might be DHSS) and those are cheap (if they still have them) and they only work with the early M11 and MX3 sport radio.
personally, im getting/have FHSS-3 for my nitro buggy and truggy and when i get a 1/10th buggy in the winter, ill get the FHSS-2.
hope this helps.