US-based automotive firm General Motors (GM) has confirmed it has received three final offers for majority stakes in its European Opel unit. The bidders are Canadian car parts maker Magna, Belgian-based investment firm RHJ International and China's state-owned Beijing Automotive Industries Holding Co (BAIC).   Canadian Magna remains the favourite to buy a majority stake in Opel, having made last-minute alterations to its bid by diluting its Russian backer Sberbank's stake to 27.5 per cent and upping its original 20 per cent stake to the same amount.