3 hatch before pool is actually pretty common in LOTV zvp. With 3 hatch, You can't really stop a cannon rush from a protoss that reaaaaaaly wants it (forge 1st, no expansion until after cannons go down), cause they will have enough mineral bank to build as many cannons as necessary until no amount of worker pulls can stop it. I dont find this to actually be that common though (since it is very all-in from the protoss), what is more common is a nexus->forge with 2 cannons and 1/2 pylons simcitied into the mineral line. You have a good shot of defending the expansions by doing the following.
A)Do not send your overlords across the map - instead park one above each expansion. If you do not have vision of your bases and cannot see that cannons have been started, you will lose.
B) follow each probe present in your bases with 1 worker. This will make it much more difficult for the protoss to easily simcity cannons in unreachable places behind the mineral, which will make the cannon rush virtually unstoppable if successful.
C)do as much damage to the cannons as possible with a 6-8 worker, which you should pull as soon as you see a cannon go down. Dont pull the workers just because the toss starts a pylon, a good toss will notice if you pull tons of workers before he actually starts a cannon, and will just cancel the pylon and abort the rush after you waste half of your mineral income chasing his probe around your base for 30 seconds. If the cannons do complete but are heavily damaged, dont sacrifice the workers! send them back to the main to resume mining, and build a pulse of lings as soon as your pool finishes (instead of queens). When the lings finish, pull the workers again to finish off the cannons, making sure that the lings are in front to tank the cannon shots.
D) If you see the rush as soon as it starts but are pretty sure it will be impossible to defend (such as when toss gets a good sim-city behind the mineral line, or when toss commits to a large number of cannons), it may be best to cut your losses. In the best case scenario, in which toss is cannon rushing one of your bases but not the other (which you can see cause you were smart and placed your overlords correctly), you can wait until just after the cannons finish before canceling the hatchery (dont bother pulling workers in this case). Unless toss guessed that you were going to three hatch before pool before scouting your base, there is a very narrow timing window in which an additional probe sent from his main after the 1s will reach your expo in time to successfully cannon before you get a few lings out, narrow enough that many toss players wont bother risking it.
wabbajack