Is it inhumane to go into adulthood sharing a bedroom with 2 siblings as well as sharing a bed with one of them?

I understand what you mean. My parents have a decent amount of money but they decided to get a small house and have 4 kids. The reason they don't want be to sleep in the room in the basement is because my mom keeps her storage bins in there but there are not that many bins and there are definitely other places to put them but my mom doesn't care. When I mention it she gets mad at me  ugh. Also I could get a job and everything and get an apartment but the apartments around me are extremely expensive cos we live near Manhattan. And the bedroom can't even fit a chair never mind another bed.

maybe you should all at least get your own bed

It all depends on what you can afford. I remember still sleeping with my mom and brother when I was 16 years old. I hated it but my mom was a single mother of 4 kids and that's all she could afford. Our queen size bed with springs. We had it for many years. Eventually my brother and I got bunk beds and she had her own. I wouldn't say its inhumane, it just happens. When you get older and work you will be able to provide for yourself. And I understand you feel suffocated because you don't have space and you need privacy. Do you parents give you a reason why you can't use the basement? Be grateful for what you have because some people don't have a place to live and their not in drugs. I was homeless before too for a short period of time.