Among the many desirable objects on offer during the 1991 Americana Week in New York - the annual high point of antiques sales - are three prized pieces. One stands in a spolight at the Winter Antiques Show: a pine blanket chest made for a farmer in the 1750s and still wearing its original coat of robin's egg blue paint. The asking price is $250,000. A few blocks away, on display at Sotheby's, is a rare Chippendale card table, created in Philadelphia in 1759. The auction house is hoping that bidding for the piece will reach one million dollars. Also on display at Sotheby's is an inlaid sofa table from the Federal period, valued at $100,000, one of the prized possessions of a collector forced by circumstance to sell his cherished objects.