Three private equity firms said Friday they were considering an offer to purchase British food retailer J. Sainsbury here. The firms, CVC Capital Partners, Kohlberg Kravis Roberts and The Blackstone Group, issued a statement to “confirm they are in the preliminary stages of assessing Sainsbury,” but said no decisions had been made whether the group might make an offer. Sainsbury operates 769 stores. Its shares on Friday climbed by 18%, giving it a market value of around $17.3 billion. The retailer in a statement Friday confirmed investor interest but offered no further comment.