Fanoffourbears...
I'd like to direct you to this lengthy thread on hydroxychloroquine
https://thecardboard.org/board/showthread.php?tid=19212
It's a nine page thread going back to March where the results of clinical trials on hydroxychloroquine were linked. As you can see, it started out promising, but once you get towards the end and the high quality randomized placebo-controlled clinical trials are reported, you can see that the results are basically null. While it is, of course, impossible to prove a negative, the results to date are incompatible with hydroxchloroquine having a clinically meaningful effect.
The limited data I have seen on Vitamin D is similar; while patients who do poorly disproportionately have vitamin D deficiencies, that turns out to be a marker of poor health generally, and supplemental vitamin D doesn't seem to make a difference. I've seen less about vitamin C, but as a water soluble vitamin, I would be surprised if large quantity supplements do much other than get excreted in the urine.
There are cheap drugs that do work and they are being used for people with COVID-19. Chief among these is dexamethasone, which results in an improvement in survival, especially amongst the sickest patients.
I think it is important to keep an open mind about drugs that could help with COVID-19, especially approved drugs that are available in generic formulations cheaply. But it is also important not to get too caught up on any one therapy. Most drugs or vitamins aren't going to work, so it should be no surprise if your favorite one doesn't.
BC