3G troubleshootings

Are you sure there's a 3G signal in your area? Check with your carrier to see if they're having a problem. (There's no "problem" of yours to fix, since your phone is capable of 4G. If people in that area with 3G phones can't get a 3G signal, their data is going at 2G speeds, which is slow enough to be painful. But your phone is using 4G speed, so the fact that a slower system - 3G - isn't working there shouldn't bother you.)