Concerning the $3,000 debt I am assuming you live in the US
If you decide to sue him in civil court you will have to look up the guidelines in your State. In my State Small Claims Civil Court is under $3,000.
Filing suit in a higher court is more complex. Depending on how savvy you are might need an attorney.
You will need some type of written documentation from him that he agreed to repay for his portion of the rent. Otherwise it will just turn into she said vs he said type of thing.
Lets say you sue for $3,000 & win a judgement against him. Based on what you are saying concerning his other debt unless he has some type of clearly identifiable assets I am not aware of bank accounts, real estate, property, vehicles, stocks, etc you wont be able to do anything with the judgement. You can get in line along with the other debtors. Cant get blood from a stone.
It will also cost you some amount of money to file in court. Might be throwing good money after bad.
You have to do what you have to do but my advice is to cut your loses & walk away. Hes an addict & this is a very bad situation for you to be in. Your health & well being are worth far more than what he owes you financially.
PS I had to eventually cut my significant loses too.