 Why do the Karna demand the conference begin in three days?
The Karna considered this to be fully neutral territory, and Earth couldn't argue the point very well. In addition, they demanded that the conference begin in three days, Terrestrial time. It would take more than a week for the Earth government to get a vessel to Saarkkad V. Earth had been caught unprepared for an armistice. The Karna pointed out that the Saarkkad sun was just as far from Karn as it was from Earth, that it was only a few million miles from a planet which was allied with Earth, and that it was unfair for Earth to take so much time in preparing for an armistice. So Earth had to get a delegation to meet with the Karna representatives within the three-day limit or lose what might be a vital point in the negotiations.
 The Karna are skilled negotiators and want to control the peace talks.
 There is an immediate threat to the planet of Karn, and the Karna desperately need help from Earth.  The Karna are hoping to disrupt Earth's supply chain.  The Karna want to make Earth look bad in the eyes of the other planets.  The Karna are skilled negotiators and want to control the peace talks.