Usually people who have these kind of boots get this process done before they are worn. A lot of people say that doing this also decreases the lifespan of the boot. Older boots have the chance of the liner inside to start to break down over time which obviously makes it hard to heat mold them. Knowing that you have used these boots for 3 seasons and 3 seasons of wear have not maintained the shape of the liner, it might be time to move on.
But on the other hand, it wouldn't bankrupt you to try. Most ski and board shops offer this service at a relatively cheap cost. Between $15 and $40. Check with the shop you bought them at and you might get a deal.
To sum up. The pros of doing this is that it might actually work and you won't have to spend big money to purchase new boots. The cons of this is that you spend $15-$40 on the molding and or you have to buy new boots.