WASHINGTON — The US Federal Reserve kept its key interest rate unchanged Dec. 13 for a third straight time, a sign that it is likely done raising rates after having imposed the fastest string of increases in four decades to fight painfully high inflation. The Fed’s policymakers also signalled that they expect to make three quarter-point cuts to their benchmark interest rate next year, fewer than the five envisioned by some economists. The relatively few number of rate cuts — which may not begin until the second half of the year — suggest that the officials think high borrowing rates will still be needed for most of next year to further slow spending and inflation.  …So far, the Fed has achieved what few observers had thought possible: Inflation has tumbled without an accompanying surge in unemployment or a recession, which typically coincide with efforts to cool the economy and curb inflation.