I've been in a similar situation. It could be that your boss feels the company has invested time and money into your "training" (even if you didn't receive any formal instruction), so they don't want to replace you and incur that cost again. Whether this sort of thinking is justified or not -- I think it's not, considering you've been there for 3 years and your training must have paid out long ago -- in practice it's clear your boss' interests aren't aligned with yours.

Given that, and given that you've already expressed your wishes to your boss, you should start looking for another job. Unless you live an area with a shortage of programming jobs and you are unable to move to a better area (very unlikely if you live in the US), I'm sure you'll find something.

Don't even mention leaving your job if you don't have a job offer from another company. In fact, if you get a good job offer you should probably accept it; using it as leverage to move within your company might work, but you should think really hard if you want to stay at your current company.

Then give him an ultimatum. Don't be headstrong if you can't find another job, but if you have that cushion, I would suggest telling him. You would like to be put in a more serious position instead of support and you feel like there should be a set date or milestone where you will be allowed to do so.

Sometimes it feels like they hold all the chips, but you can quit at any time and they will be sh*t out of luck. Don't let them forget it and your bargaining will be easier.

Ask for a raise or new responsibilities. Tell them if they want you to wait it out that you need to be compensated for your skills languishing. Take the higher compensation and start working on a side project, invest all the money that you make over your current compensation into launching that side project when you are ready.