VIENNA—Britain, France, and Germany are pressing ahead with a U.S.-backed plan for a resolution by the U.N. nuclear watchdog’s board criticizing Iran for curbing cooperation with the agency, despite Russian and Iranian warnings of serious consequences. The International Atomic Energy Agency’s (IAEA) 35-nation Board of Governors is holding a quarterly meeting this week against the backdrop of faltering efforts to revive Iran’s nuclear deal with major powers now that U.S. President Joe Biden is in office. Iran has recently accelerated its violations of the 2015 deal in an apparent bid to raise pressure on Biden, as each side insists the other must move first. Tehran’s breaches are a response to the U.S. withdrawal from the deal in 2018 and the reimposition of U.S. sanctions that had been lifted under it. The latest breach was to scale back cooperation with the IAEA last week, ending extra inspection and monitoring measures introduced …