All are good cameras and will produce decent images, but my advice would be to go for the EOS 400D. This is based purely on my own experience of owning and using a Canon DSLR (EOS 20D) and is not meant to start yet another pointless debate on Canon vs Nikon.
Its a camera that you will not feel you have out-grown in a hurry, plus you will have access to the best range of lenses available.
My best advice is to go into a shop and handle all three cameras plus some others, at the end of the day you will choose the camera that feels right for you and thats the most important thing.
Good luck & let us know what you decide on.