There is a good book on being water logged out there. You actually don't need a ton of hydration to run 3 miles. Think about it, bipeds began walking to chase Antelope across the african prairie in 50 mile endurance quests. (half kidding cuz its you Brett)
Extra water will only way you down. Becoming comfortable in your reserve water can help you weigh less at race time. glug glug glug is actually what kills runners, being so water logged and drinking more at every station while under exertion is what causes many of these marathon collapses. Its not under hydration. some carbs today, not a ton, low sodium, (again to keep water weight down), and stick to fruit and easily digestible things on race day.
Take a big dump before, seriously. There is an art form to squeezing out every poke of dead weight brown matter, you alternate back and forth from leaning back, to grabbing your ankles and push. Then push through the lean back. Coffee or tea can help that around 1 pm.
The human body can function just fine beyond what we consider thirsty. I mean....most people aren't going to do that because its best to feel healthy, not dry mouth. But definitely don't go crazy on the hydration or you will be bloated and heavy. Its only three miles,
you need to relax. too. my first race I about jittered all my adrenaline out before the race. I was shaking with anticipation when I woke up. You got to meditate (pray), relax, don't think about it. Go to the park with your kids or play a video game like many athletes do. Its a good distraction. Read a book, or THE BOOK, could be a good idea.
If you start overthinking this you are going to nerve yourself out. breathe deep through the stomach. Set really reachable goals and just have fun with it. You are competing with yourself and even in a 3 mile you want to conserve at the beginning and not get sucked into the lead pack.
good luck.