They have not settled on an exact number, but personally, I think it's pretty safe to assume it'll be more than 3. They also have not decided if you will be able to purchase more. However, every pledge level above Watcher comes with an additional character slot.
Related threads:
https://www.pantheonmmo.com/content/forums/topic/3700/character-slots/view/page/1
https://www.pantheonmmo.com/content/forums/topic/3780/character-slots
https://www.pantheonmmo.com/content/forums/topic/6026/character-slots
https://www.pantheonmmo.com/content/forums/topic/3051/character-slots-in-game