Sounds good so far Randy. It read like your problems stemmed more from trying to calibrate each sub at 75db rather than something less to account for the "coupled" output.

SMS on either both or one 350v3 is what I would attempt first. When you set the SMS filters also try it with one 350v3 if you are having trouble cutting the peaks. As you pointed out, with multiple subs cutting output on one sub can result in increased peaks elsewhere because of the way the sound waves from the 3 subs are reinforcing or cancelling each other.

Also if setting sub level at 70db still results in combined sub output close to 80db you will need to set the subs to say 68db.

If you set it up right, you may be able to do away with the SMS entirely though I find its a great tool even when not connected.

PS I like your new avatar but she doesn't look like you. \:\/