That was me! I did an airstart on my Magnum with a central full J (Experimental motor) and 2 sideboard H180's. They lit a bit late, but still gave plenty of extra boost. Also, for the record, I have launched my Magnum (10 1/2 pounds dry) on an I284, and it only went 800 feet.
I would highly recommend that you NOT light the two sideboards on the ground unless you also light the central motor on the ground. Two very bad things can happen (and probably will, given how little power you will have on the boost). First, one of the motors can light later than the other, or not at all. The former will send your rocket off at an angle, far away from you, and the central motor will only send it farther. The latter will be very bad for all involved, because your rocket will probably arc over severely, hit the ground under boost, and then light the central motor. I would be reluctant to allow this configuration if I were RSO.
Second, the timing is critical. 2 H242's are going to burn for a very short time, and they will be carrying the entire weight of the unlit central motor as well. You'll get signficantly less boost out of those motors than you expect, because of the extra weight they are carrying. So, the timing of lighting the central motor, during the boost phase, will be critical to probably within a half a second. Your rocket will go a few hundred feet, and in just 1 or 2 seconds, start to arc. Your central motor will light at some point during that arc, and you'll be chasing your rocket as it goes toward the horizon.
Just my two cents.
David