Single-family home prices picked up for a third month in a row in April, suggesting the recovery in the housing market is gaining traction, a closely watched survey showed on Tuesday. The S&P/Case Shiller composite Index of 20 metropolitan areas gained 0.7 percent on a seasonally adjusted basis, topping economists’ expectations of 0.4 percent. On a non-seasonally adjusted basis, prices fared even better, rising 1.3 percent. Just three out of the 20 cities in the index saw declines in April on a seasonally adjusted basis.