I don't believe it is possible to have an authentic
tension setting with three gaps, unless the outer two of those three gaps were to have bridges underneath them.
But this may not provide sufficient support for the center stone, which would hang in the balance.
I received a beautiful engagement ring with 3 diamonds that are, I believe, tension-set (I definitely know that the center stone is). The ring is platinum and from Verragio. I have gotten so many compliments on the ring, and it is the prettiest ring I have ever seen. I have heard that to have the "safest" tension setting, platinum is required.
I knew the model at TC for a while (LINK) but how does the Boone one look like?
Tension mountings in traditional metals routinely come with sidestones (LINK) but "three stones" (with the side stones larger than an afterthought) I have yet to see.
It is a thick flat band with three small stones simulatneously held in tension. http://www.boonerings.com/htmpages/tristone.htm It needs a lot of metal to get it stiff enough to be able work. In theory, all three stones shouldn't share the load. There's a lot of measuring and careful cutting of each slot for its particular stone. The trick is to make the middle one where it could not escape even if it is held with less tension than the others. Making one with 3 different sized stones or one big and two smaller stones like the wraparound ring in the picture above would be very difficult to do as a tension set. The helical nature of that style doesn't allow as much tension as if it were straight on. The alignment is a lot more difficult as well because the ring takes on a different helix angle as it is opened to set the stones. I would suspect that there are bridges holding the small stones in place. They would also align the ring for the larger stone, which would be held similarly to a tension set.