Let me clarify my concerns a bit. I had the Spatial M1's sounding really good in an 11' x 20' x 7-8' (sloping ceiling) space. They were pretty close to the sidewalls - 8" doesn't sound too far off. But I had plenty of space to pull them 4-5' off the front wall. I probably did try them at 3', but definitely had them further out once I had dialed them in. 3' would still sound good, I think, just not as good as more space would sound. So as long as you can live with the 36" rather than the 30" you mentioned above, it could work. But note that the distance from the side walls impacted the amount of bass presence, so tweaking the sidewall distance was important to dialing in the sound. With your room dimensions and placement constraints, you just won't have much flexibility at all to tweak distance to sidewalls or to front wall to optimize to the sound you prefer. You could easily find that you need to bring them further out into the room than you want in order to sound satisfying. Or need to pull them further from the side walls than you say you have space for. Not that they wouldn't make music without this tweaking, but I'm betting you would end up trying some different placements, and there's a decent chance you won't be satisfied within your stated criteria for distances.
But another concern with your situation is the room depth behind you and general volume. I had way more space behind me (roughly 8 feet) than you're going to have. If you're sitting close to the rear wall, you'll get a lot of room boundary reinforcement in the bass. And with those double 15's, I wonder if it will overload your room on the bass front. Sure, the OB bass should deal as effectively at minimizing room modes as any system out there could, but the overall volume of bass might still be off if they're designed to fill a larger volume room. With the X series you'd have the ability to tweak bass settings, but with the M series I think it's just a set passive crossover, isn't it? So I agree the choice of M3 vs M5 should be carefully considered.