HONG KONG—Shares of Macau casino operators on Wednesday shed as much as a third of their value, losing about $18 billion, as the government kicked off a regulatory overhaul that could see its officials supervising companies in the world’s largest gambling hub. With Macau’s lucrative casino licences up for rebidding next year, the plan spooked a Hong Kong market already deep in the red after Beijing’s regulatory crackdown on sectors from technology to education and property that sliced hundreds of billions of dollars off asset values. Wynn Macau led the plunge, falling as much as 34 percent to a record low, followed by a 28 percent tumble for Sands China. Peers MGM China, Galaxy Entertainment, SJM, and Melco Entertainment all fell heavily, taking the drop to HK$143 billion ($18 billion). U.S. casino companies also fell for the second straight day, losing as much as $4 billion in market capitalization on …