Although we know of the bloody conflict between the sunni and shia, which is fought even today, I know little of the relationship of the Ibadis with the 2 other sects.
Ibadis are generally not even well known to Shi'a and Sunnis - their isolated position in Oman and the traditional non-confrontation behaviour of the Ibadi faith has led to them being a sort of discrete branch of Islam.
Source: Valerie Jon Hoffman - The Essentials of Ibadi Islam