Full-scale SST designs are almost always quads because there have not been any turbojets or low-bypass fans with enough thrust to power a tri-jet. In its day, the GE4 was the world's most powerful engine by a significant margin (excluding the competing P&W JTF17A) and the B2707 still would have needed four.
Most SSTs on the drawing board are in the 750,000-pound class, and given the thrust-weight ratio of 0.4 found on Concorde and the B2707, you'd need engines with around 100,000 pounds of thrust for a tri-jet SST. I'm not sure that any engine core available (e.g. GE90, Trent 800) could scale to this much thrust in a low-BPR application. A quad with around 75Klbs per engine seems much more manageable.
In addition, given the greater flow instability of a tail-mounted inlet, I'd imagine that shockwave management and unstart prevention would become a serious issue, especially in an outboard engine-out situation. The added structural weight of a tail-mounted engine would also be an unwelcome feature.
Keynes is dead and we are living in his long run.