I have tenderloin for beef Wellington that I'll make in 3 days - how do I store the meat?

I don't know if much moisture leaves the meat through evaporation from more than just the very outside, even with a longer wait. I would be surprised if this made a big difference to the quantity of juices released during cooking, so intuitively I would not share your worry about the steak getting too dry.

However, I would worry about an uncovered steak getting dirty, absorbing unwanted smells or flavours, or having some kind of pathogen or mould growing on it. If you want to give it a go a good way to do it with less risk would be to put it in an airtight Tupperware container big enough for there to be space around the steak, with some kitchen roll in the box to absorb any moisture released. Changing the paper if it gets damp should be enough to keep the air inside the box dry.