In Tennessee, the state insurance commissioner approved a 36 percent rate increase for the largest health insurer in the state’s individual marketplace. In Iowa, the commissioner approved rate increases averaging 29 percent for the state’s dominant insurer. Health insurance consumers logging into HealthCare.gov on Sunday for the first day of the Affordable Care Act’s third open enrollment season may be in for sticker shock, unless they are willing to shop around. Federal officials acknowledged on Friday that many people would need to pick new plans to avoid substantial increases in premiums.