One way to do it is to use a boolean flag. Call it OkToRunMotors or some such. Set it true before your for loop. In the loop, check whether each sensor is detecting a range of less than 30. If it does, stop the motors and set the flag false.
After the for loop, if OkToRunMotors is still true, run the motors.
n.b. don't run the motors from the for loop itself as you are currently doing - per your requirement, you need to have established that all sensors show the way clear before you can do that.