Ahhh, I know how you feel. 3 days with no shower won't feel great. Esp if you're outside most of the time...
I think your best bet is to do as everyone said above. Maybe wear your hair down Day 1 but tie it back later. Wear a hat/bandana. Bring a big bag, knapsack, purse, etc. Maybe look into getting those Clean and Clear oil absorber papers (they're GOOD!). Bring some face powder, some lip gloss, a small brush, some food. You can buy those tiny cases of face/hand wipes and bring that to if you want to clean your face. Little small thing of perfume/body spray. So what if you carry a purse? All these things are tiny.
Also, maybe get that Maybelline Overtime Lash Tint. I haven't tried it yet myself but apparently it stays on for 3 days!
In the end, don't let this get in the way of you going. When else are you going to have the experience to do something like this again?