A driving assistance device of a vehicle connected to a trailer, a vehicle system including the same, and a method thereof are provided. The vehicle driving assistance device includes a processor that determines a driving situation and a braking situation of a vehicle towing a trailer based on vehicle's internal signals and determines an amount of rear wheel steering control and an amount of braking control based on the driving situation and the braking situation of the vehicle and a storage storing the amount of rear wheel steering control and the amount of braking control, determined by the processor, and the vehicle's internal signals.