Anyone know if it can come in 3" or is it only for 3 1/2" chamber and if not could "00" buckshot prove the best for coyote hunting with a
12 Ga.

It is available in 3" and I believe 4 buck is better coyote medicine than 00 buck. No matter which you use, be sure to try a few different chokes in your shotgun, as you may find a more open choke to pattern the large pellets better.

I got to agree with howler on the #4 buck, but the dead coyote is hard to beat IMHO. If you have a turkey choke i'd try that first, it works well with my 870.

I've used the Hevi-Shot Dead Coyote 3" loads, and they definately do a good job.

Also look for a heavy load of Copper plated 1 5/8 ounce BBs. They work very well as well.

I wouldn't use the OO buck, 4 buck hits the the hammer of Thor.

Just make sure to spend a little time experimenting with different chokes pattern testing.

That gives me a guideline. I got a few months to experiment for I usually don't target coyotes until after deer season.