HONG KONG — A Hong Kong court sent young activist Joshua Wong and two other student leaders to prison Thursday for their roles in huge pro-democracy protests nearly three years earlier, in the latest sign that tolerance for dissent is waning in the Chinese-ruled former British colony. The High Court overturned an earlier verdict that let Wong, Nathan Law and Alex Chow avoid prison, agreeing with prosecutors that the original punishment for joining or leading an unlawful assembly that sparked the protests was too light. They were immediately taken to serve their sentences of up to eight months, which have the added consequence of blocking each of them from seeking public office for five years.