You can find Cerberus in Hades' layer. Right after you pass the River Styx. :)
the cerberus is a 3 headed dog on mythology island on poptropica
on cerberus's face
he is in the under world
go to Apollo's temple and talk to the statue with the reed pipe and pass the test then you have the tune to put cerebus asleep
In the Underworld after crossing the river of dead.
In Poptropica, you don't fight Cerberus (the three headed dog). You play a tune on your pipes to make him fall asleep.
If you mean Cerberus, he is in Hades' Realm after you pass the River Sytx. Play the tune: Blue, Green, Yellow, Blue, Red, Green on your reed pipe and he will fall asleep. Then, you can collect the Sacred Item: Whisker From Cerberus.
he isn't in a room. he is near river styx
Play the song one of the Muses gave you on your reed pipe. This will make Cerberus fall asleep and you can get its whiskers.
You just have to get into the Underworld, get past the River Styx, then put cerberus to sleep with your pipe, and grab a whisker (which is needed) from it.
You play him the song you learned earlier on the reed pipe.