If I knew my life was ending in 3 days I don't think I'd do anything grand. It's not like I'm a hero or a celebrity. It's not like I have the money to do anything audacious or that I would want to impart my will on everyone for an unknown amount of time. I would likely watch youtube, hang out with my cat a bit more, and not tell my friends of my circumstances. I don't think I'd leave a letter either.
Perspective on these kinds of things can change with time, of course, but I've felt similarly for a while now. It's always like you're just drifting through life with little control over yourself, little to impart on others that feels impactful, and little to achieve what makes you happy in the moment without facing large consequences.
If I had 3 days, I would keep being me while maybe listening to the loudest music possible and maybe I'd start writing poetry again. Surely an event as big as your death will make the ideas start flowing? At least, that's what I feel most people would think. I've had a few near death experiences and one recently that made it seem like I will die here shortly (not including my depressive episodes), and nothing changed in that regard. The only change that happened for me, was a slight recontextualization of how I think about how my actions affect those around me, but even then it doesn't mean I think before I speak (sorry to those who know who they are).
In the end, while I think I know how I would act and feel with only having 3 days left, it could be that the fact that I wasn't certain about my end means that I continued on as normal until I keel over because there was a chance. If there was no chance, would that finally be the thing that would sway me to actually change the way I behave? Would I be able to study? Would I be able to focus on topics that don't immediately grab my attention? Would I try to go out and meet new people?