The Merchant of Death discussion

The Merchant of Death is a great fantasy book but I wouldn't reccommend it if you don't like fantasy or cannot read long books. It is also in a series, so if you liked this you should read them too. In the beginning of the book Bobby Pendragon's uncle tells him that some people need his help and takes him to a subway. There he is attacked by three-headed dogs and his uncle leads him to a door with a glowing star on it. When he jumps through it he is picked up by a magical force and around him seems to be Time itself. When he gets out of the mysterious passageway he lands in a new place called Denduron.
If any of this is wrong please tell me, I read the a long time ago.

I dont believe it is a three headed dog, but the rest is correct

I may be thinking about Cerberus