WASHINGTON—The U.S. economy grew at a record pace in the third quarter, fueled by more than $3 trillion in pandemic relief, the government confirmed on Dec. 22, but appears to have lost momentum as the year draws to an end amid raging new COVID-19 cases and dwindling fiscal stimulus. The economy plunged into recession in February and remains 3.4 percent below its level at the end of 2019. The United States is struggling with a resurgence in new coronavirus cases, with more than 17.78 million people infected and over 317,800 dead, according to a Reuters tally of official data. State and local governments have reimposed restrictions on businesses, undercutting consumer spending and unleashing a fresh wave of layoffs. The grim situation has been worsened by Congress’ delay in providing additional fiscal relief for struggling businesses and the unemployed. “A few tenths more growth in the third quarter won’t be nearly …