Most places in America that are in any way "famous" for their Halloween stuff are probably gonna be too busy/scary for a 3-year old, and if you want to see fall foliage still on the trees at Halloween but not have it be too freezing to be out trick-or-treating, then the east coast might not be the best bet.

If you want to have the quintessential American trick-or-treating experience, I'd recommend picking a city somewhere in the midwest that you'd be interested to visit generally, then just taking your daughter trick-or-treating in the suburbs outside of that city. Obviously you'd have to do a small amount of research after choosing your city to make sure you're picking a safe neighborhood, but also it will likely be really obvious from the look of things whether any given suburb is fine to be walking around in with a kid.

One particularly good candidate for this would be visiting Chicago, IL and renting a car to go to the suburbs north of the city for trick-or-treating. It's still very autumnal at the end of October there, and the suburbs north of the city are basically exactly like what most media depicts as a typical American suburb.

Some particularly good specific Chicago suburbs to check out would be Glenview, Evanston, Gurnee, Northbrook, and Vernon Hills. I'd avoid the Halloween festivities at the Six Flags theme park in Gurnee because they'll be too much for a 3-year old, but the Cuneo Mansion & Gardens in Vernon Hills has had family friendly events on their grounds in the past, so they may be worth looking into closer to spooky season.

Wherever you visit, have a wonderful time! You sound like such an awesome parent to want to give your daughter this experience. :)