I'd say that if the niacin is giving you satisfactory results over a 3 years trial and you've had your liver enzymes evaluated lately with normal results, "stay with the demons you know" (as the saying goes.)
I think, of the three choices, that policosanol is the one that is least likely to cause any liver problems (or any problems) for you. Whether or not it will give good results is up for grabs. You can PROBABLY add it to the niacin for a trial.