Easily, if it’s a large enough backpack. For 3 days of clothing, it doesn’t even need to be large. I mean what do you need, some assortment of care products (available in small packs, usually, so don’t take a ton of space), maybe a razor and hairbrush, toothbrush (negligibly tiny anyway) and then 3 days of clothing don’t take that much space unless you change your pants (not the british kind) or sweatshirt every day. And even then a large-ish backpack could fit it.

I went on a 5 day trip with just an average size backpack and laptop bag, and that included bringing the laptop. And headphones. Clothing doesn’t take that much space if you fold it and compress a bit.