Are the tons long tons or metric tons? I will assume metric tons of 1000 kg.
So the mass of the vehicle is 3000 kg.
50 km/hr = 13.89 m/s
Assume that the total force during the decelleration is constant.
Then the KE of the vehicle is 0.5 3000 13.89^2,
If we have a constant decellerating force f it does work 50 f, which
is equal to the initial KE, so:
50 f = 1500 13.89^2,
or: f=30 13.89^2= 5788.0 N
So the breaking force is this less the tractive resistance of 150x3 N or