By Jessie Pang and Joyce Zhou HONG KONG (Reuters) -Three Hong Kong pro-democracy activists were arrested on Sunday, just before voting began in a “patriots only” district election that has marginalised formerly popular opposition figures in the city amid a national security clampdown. The pro-China government has been seeking to boost turnout, as some observers see large numbers spurning the polls, in contrast to the last council elections in 2019, during Hong Kong’s mass pro-democracy protests, which drew a record 71% turnout and a landslide victory for the democratic camp. Police arrested th…