Developers in Montgomery County soon will see their first piece of action in their quest to transform White Flint into an urban economic engine that many say will outshine downtown Bethesda as the new model for D.C.’s outlying communities. Three developers building on a total of 35 acres on Rockville Pike in North Bethesda are expected to submit their sketch plans for their sites, which include residential and office space, hotels and retail, to the county planning board in late January.