Federal regulators were considering a new rescue for Citigroup on Sunday, a step that could mark a third leg of the government’s broader efforts to bolster the nation’s financial industry, according to people briefed on the plan. Under the proposal, the government would shoulder losses at Citigroup if those losses exceeded certain levels, according to these people, who spoke on the condition that they not be identified because the plan was still under discussion. If the government should have to take on the bigger losses, it would receive a stake in Citigroup.