I want to be sure I understand the plan. What's certain at this point is an array of 3" fullrangers crossed to side firing woofers or subwoofers at 250hz. The decision is whether to use a side firing array of 6" woofers or a single 12" stereo sub. If I understood this correctly there are some issues to resolve before you buy drivers or start cutting mdf:1) Will the array of 3" fullrangers carry all the frequencies above 250hz, or will they be crossed to a tweeter array? Roger Russell of McIntosh Labs fame has proved an equalized full-range array can work, but there's still a question about whether it's as good as the more common practice of using an array of small to mid-size woofers with a separate array of ribbon, planar or dome tweeters. The consensus around the web is that the latter approach is better. See the link to Russell's web page below.
2) Crossing over to side firing woofers at 250hz raises some concerns about the use of side-firing versus front-firing woofers and about the appropraite crossover frequency. I'm out of my element here and just repeating what I have read, so does somebody else have some comments about this?