Keep in mind that a speaker's loudness level dies down much more quickly outdoors than it does indoors, following the inverse square law. Say you have your speakers set such that the level at your position on the deck, x meters away, is 75dB (quite loud). A person twice as far away from the speakers as you, 2x, would hear it as 69dB, about as loud as a busy street. Double that distance again to 4x, and you have 62dB, which is about the level of a normal face-to-face conversation.
I believe your best option is to hang the speakers under the eaves pointing at a downward angle such that the drivers aren't firing directly at any neighbor's house. This way, the ground (hopefully something soft like a lawn) will take care of the direct reflections. If the neighbors hear anything, it will be toward the bass end of the spectrum, and since the M3s are neither boomy nor delve too deeply into the bass regions, you should be OK there as well.
