Administration officials are arranging a plan for a multipart infrastructure and economic package that could cost as much as $3 trillion and fulfill key elements of President Biden’s campaign agenda, according to people involved in the discussions.The first proposal would center on roads, bridges and other infrastructure projects and include many of the climate-change initiatives Mr. Biden outlined in the “Build Back Better” plan he released during the 2020 campaign.The package would be followed by measures emphasized on education and other priorities, including extending the newly expanded child tax credit scheduled to expire at the end of the year and providing for universal prekindergarten and tuition-free community college. The packages could face a hard path through the narrowly divided Congress, and Democrats aren’t all in agreement on how they should move forward with their spending and tax proposals. Mr. Biden is expected to be briefed on the details of proposals this week, and the people warned that the strategy is preliminary. Mr. Biden would require to sign off on the legislative for it to move forward.