Google and Pinterest have both disclosed plans to trim scores of tech jobs in the Bay Area in layoffs that are poised to jolt workers in three of the region’s cities. The plans for employment reductions by the two high-profile companies arrive on the heels of a December upswing in Bay Area tech hiring that was primarily concentrated in the South Bay. The layoff decisions by Google and Pinterest were revealed through posts by the state Employment Development Department on its website. In December, the Bay Area added 200 tech jobs, according to seasonally adjusted industry totals that Beacon Economics derived from the official monthly report from the state EDD. The South Bay added 1,800 tech positions in December.