validates_presence_of :field1, :if => :field_required?
validates_presence_of :field2, :if => :field_required?
validates_presence_of :field3, :if => :field_required?

def field_required?
  if self.field1.blank? && self.field2.blank? && self.field3.blank?
    return true
  else
    return false
  end
end