SHANGHAI—China’s plans to launch a new exchange in Beijing, announced by Chinese leader Xi Jinping on Thursday, boosted shares in Chinese brokerages, but knocked down Shenzhen start-up board ChiNext and shares of Hong Kong’s bourse, amid fears of rising competition. Although China’s securities regulator said the planned Beijing stock exchange is based on the city’s existing New Third Board, and complements Shanghai and Shenzhen bourses, some fear a rivalry for listing resources is inevitable. “The Beijing stock exchange has equal footing with Shanghai and Shenzhen bourses. If it prospers, the three will share the market in tripartite confrontation,” wrote Rock Jin, economist and CEO of investment adviser PopEton. Although it was good news for the economy, it boded ill for the market in the short term because “after all, it diverts capital away from Shanghai and Shenzhen markets.” Xi unveiled plans for the new exchange in a video address at …