What are the only three animals known to have blue tongues?

Blue-tongued lizards, Chow-Chows, giraffes.
http://www.outback-australia-travel-secrets.com/blue-tongue-lizard.html
http://www.mnn.com/earth-matters/animals/stories/why-do-some-animals-have-blue-tongues

USELESS INFORMATION #43: There are only three animals with blue tongues -- the black bear, the Chow Chow dog and the blue-tongued lizard.

http://members.datafast.net.au/carolf/useless.htm

http://animals.sandiegozoo.org/animals/giraffe

A giraffe's tongue is 18 to 20 inches (46 to 50 centimeters) long and blue-black. Some people think the color is to keep the tongue from getting sunburned. So if the black bear also has a blue tongue, that makes 4 animals. I haven't been able to find any article nominating black bears having blue tongues.

Black bears do NOT have a blue tongue. There are plenty of pictures on the internet that clearly show a black bear's tongue to be pink.

That means there are at least 5 animals that do have a blue tongue: blue-tongued skinks, Chow-Chow dogs, giraffes, okapis and impalas.

Just note that an impala's tongue isn't as bright blue as the others. It's a bit more greyish-blue.