WASHINGTON—U.S. retail sales fell for a third straight month in December as renewed measures to slow the spread of COVID-19 triggered job losses, further evidence that the wounded economy lost considerable speed at the end of 2020. The downturn in sales reported by the Commerce Department on Friday is, however, unlikely to push the economy back into recession, with other data showing production at factories accelerating last month. There is also cautious optimism that nearly $900 billion in additional pandemic relief provided by the government at the end of December will offer a backstop. The ebbing economic momentum, which appears to have spilled over into the new year, could persuade the U.S. Congress to agree to President-elect Joe Biden’s ambitious $1.9 trillion fiscal stimulus plan, which includes bolstering the response to the virus and direct relief to households and small businesses. “That should make Congress more willing to deal on …