As it was explained to me, the reason for the 3ft from the wall has to do with reflection timing. If the reflections off the front wall reach your ears too close in time to the direct wave from the speaker the sound is muddied and smeared. The closer to the front wall, the more the smearing. 3ft separates the direct and reflected waves enough that they are perceived as two distinct sounds. Not only does this eliminate the smearing, it provides a sense of spaciousness to the sound.
I have experimented with my open baffle speakers being 3ft from the front wall, closer and farther away. With at least 3ft, the sound seems to come from the entire front wall for width and height (soundstage is wider and higher then the actual speaker placement. And this with the speakers only 5ft apart (try that with box speakers). For depth, the front wall disappears in that the soundstage seems much deeper than the distance between the speakers and where the wall should be. In a larger room (20ft x 25ft) I found the 5ft from the front wall and 7ft apart sounded best.
However, moving them closer than 3ft had a number of deleterious effects. In addition to smearing and less clarity, I found that moving the speakers closer to the front wall collapsed the soundstage. The closer to the wall, the more the soundstage shrunk.
If you can use the single sub on it's side between the speakers, and have the sub at least 3ft from the wall then go for it. This will give you the best performance with a single OB sub. If you put the center channel speaker on top of the sub, put some kind of damping material between the two.
If there is no way you can get the 3ft distance from the front wall for the sub, the second best choice would be to use one of the 12" sealed servo subs. You could place this wherever it worked best (sub crawl). I used this method for six months after building the Wedgies while saving up for the OB bass units. However, do be aware that this is a compromise. You will get your bass but it won't quite mesh properly. After having listened to both types of bass, I am of the firm opinion that OB bass is the only way to properly integrate bass with OB speakers.