Rhode Island is among three states with the nation's highest sales-tax rate, but the group will soon be getting some company. According to a survey issued yesterday, Rhode Island, Mississippi and Tennessee all levy sales tax at a rate of 7 percent. That is the top state sales-tax rate in the country, according to the survey by CCH, a publisher of tax information for accountants, lawyers and others. But another state will soon join them: New Jersey recently boosted its rate to 7 percent, from 6 percent. The new rate takes effect on Saturday, according to the New Jersey Division of Taxation. Rhode Island's General Assembly last month resolved to form a special legislative commission to study "all aspects of the state sales tax." The commission will focus, in part, on the following areas: Evaluating the business and economic impact of an adjustment to the sales-tax rate.