Why do they say the Jesus story is like Mithra's and Horus'?

Mithra and Horus weren't born on Dec 25th because the Gregorian calendar didn't exist back then. No places where I read said that they died for three days, were born of a virgin, had disciples, taught at 12. If so, then give me sources for that. There is also nothing about Dysonidus that's similar to Jesus, beside the fact they're deities/godmen (sp?).