(Corrects to say arrests were made by Saudi and Kuwaiti authorities in para 1, one to be extradited in para 4, edits headline) DUBAI, July 7 (Reuters) - Three Brothers have been arrested by Saudi and Kuwaiti authorities for involvement in a suicide bombing by a Saudi man on a Shi'ite Muslim mosque in neighbouring Kuwait last month, Saudi State news agency SPA reported on Tuesday. The ultra-radical Sunni Islamic State group said it carried out the attack, which killed 27 people and appeared aimed at stoking sectarian hatred in the energy-rich Gulf. The three Saudi brothers, who were **t identified, were "parties to the crime of the sinful terrorist bombing that targeted the Imam al-Sadeq mosque in Kuwait", SPA quoted a security spokesman for the interior ministry as saying.