The answer to whether it is sufficient or not to leave three loan packages locked on the front seat of the car, having to choose between a yes or no answer, would be no, it isn't sufficient, in order to protect the packages, leaving them securely locked on the front seat of the car.
Of course that their safety depends on some factors, like, for example, if the neighborhood is considered safe based on the rate of car robberies. Even so, leaving something valuable exposed on the front seat of a car is usually taking a risk. In this situation, even if the neighborhood is considered safe, a car robbery is always something possible. Due to the possibility, the safest thing would be putting the packages in the trunk of the car or taking them along. Being sufficient or not depends on factors like the robbery rate, but also depends on circumstantial events that can't be trusted. If the loan packages are very important, they shouldn't have been left on the seat of the car. Therefore, leaving the packages on securely locked on the front seat of the car isn't sufficient to keep them safe.