The National Assn. of Realtors said that further declines in mortgage rates helped boost sales of existing single-family homes to 3 million units for the month, the highest level since last April, when 3.03 million units were sold. It was the fourth consecutive month that the sales rate has risen. Sales were up 17.3% in the Northeast, 9.7% in the Midwest and 0.9% in the South, but dropped 5.8% in the West. The survey found that the median price of a previously owned home increased 3.1% in January to $74,300. The median price, which means half of the homes sold for less and half for more, was up 5.1% from January, 1984.