Northern Mariana Islands U.S. Workforce Act of 2018 (Sec. 3) This bill extends by 10 years, through 2029, and revises the immigration transition program for the Commonwealth of the Northern Mariana Islands (CNMI). The Governor of the CNMI must submit to the Department of Labor, and Labor must approve, a plan for using fees collected for vocational education. Labor must report on the number of asylum seekers it anticipates at the end of the transition program and its efforts to prepare for the asylum seekers. The bill revises the Commonwealth Only Transitional Worker permit system, including to limit the validity of a permit to a one-year period, at most. Employers petitioning for one or more such permits must pay a specified fraud prevention and detection fee. Before an employer's petition for a permit may be approved by Labor, the employer must apply to Labor for a temporary labor certification that confirms (1) there are not sufficient U.S. workers in the CNMI who are able, willing, qualified, and available to perform the work involved in the petition; and (2) employment of the nonimmigrant worker will not adversely affect the wages and working conditions of similarly employed U.S. workers. Workers under permits must be paid the highest of CNMI minimum wage, federal minimum wage, or the prevailing wage for the occupation. The bill provides for renewal of permits and three-year permits for workers permitted since FY2015. The bill caps the number of permits that may be issued each fiscal year. The cap is lowered each year.