it could be the motor brushes being stuck in the brush holders, i've seen this before. You can use a electric motor cleaner(do not use anything else), and spray down the brushes and use a tool and grab the brush lead and pull them out and let them snap back in. Do this a couple of times to see if it helps. Also look at where the heal of the brush is in relationship to the brush holder bracket. If they are worn out it will be even with the back of the bracket or down inside it, if you see this then replace the brushes. If they are protruding out past the rear of the bracket holder then they are probably not worn out and just sticking in the holder. Also check the armature surface, if you see burning where the brushes ride then that is a sign that they might be sticking and arcing too badly burning the armature bars.

One other thing to consider is on these 3 wheel trucks there are usually 2 sets of contactors for each side, a right and left forward and a right and left reverse directional contactor.

check the contactor tips and see if they are burnt or melted. Check the armature of the contactor and make sure it is not worn out, this can cause then to bind and not close completely. If you find this then the contactor will have to be replaced.