I don't think there was ever any real canon. Each film is essentially it's own continuity. The second film is considered a prequel to the first, but being a loose adaptation of the Ronald Defeo story seems to be the only reason it's called a prequel.
If I recall correctly AMITYVILLE 3D actually includes a disclaimer at the end saying it's not affiliated with any previous film adaptation. But it has since been labelled the third entry, especially after a film adaptation of the John Jones book AMITYVILLE: THE EVIL ESCAPES included a "4" in the title.
http://www.freewebs.com/demonictoys/

No wonder people say it's confusing!

1 and 2, I
think
, are compatible with one another, as the real-life killer is never mentioned.
3 mentions the real-life killer. It's its own beast.
Amityville 4 and 6-8 could be considered their own canon. 4 ignores the destruction of the house in 3 and involves its own 'cleansing' of the house. 7, once again, claims a different fictional murderer was responsible for the killings in the house. All four of these films involve a piece of the Amityville house 'escaping' and tormenting a new family.
The Amityville Curse should be considered its own beast, since it doesn't even take place in the same Amityville house.
Everything after the remake should probably considered one-shots. All have something different to say about the fate of 112 Ocean Avenue, if I remember correctly. In Haunting, it's now a suburban home that looks nothing like the original. In Asylum, the house was supposedly torn down in order to build an asylum.