I love tri-jets myself... i think they are the most interesting and classy looking designs, but
would be used for fairly short trips. Im not sure their would be too many advantages to the trijet design in this case. It did work for the BAe-146, but one could argue in many ways, the BAe-146-300 didn't as well as it could have because it hard a hard time competiting with DC-9-30s and B737-200s. The clear advantage the BAe146 had was its very short take off and landing ability.
Maybe this is the angle you could market a tri-jet rj on? Unfortunately, it is going to be a long time before if ever, we see something like the good old 727 produced again. I think the tri-jet's design best hope is still for long-haul flights.
Here's a thought. The 777 and A380 will have similar engines. Not quite but sort of in the same way the 767, MD
-11 and 744 have similar engines. How about a trijet 450 seater using RR
trents, or the other 777 engines? Ultra long range, so its ETOPS free, yet less maintenance than a quad, and should be able to get off shorter runways that the 777-300.