Three acres is not the wide open spaces, and a marsh is water. Rule 1 is to not shoot over or at a body of water, due to ricochet concerns. I'd find a safer place to shoot with a lot more distance to the next house and a good backstop.
Are you certain there is no one behind your property? That AR-15 has a one mile or more range. Go walk straight back one or two miles and see if there are any trails, roads, whatever, then think about how you are going to clear that entire downrange area each time you want to shoot off the back porch.