The reason aircraft had 3+ engines in the past was because it couldn't be done with 2. We're not just talking about fuel efficiency and reliability here, we're talking about engine failure at the most critical phase of flight, taking a 707 or 727 in the air and losing more than one power plant past V1 is a good reason to put it back on the ground and take your chances if you are anywhere near MTOW.
You're talking about adding a lot of complexity to something that should be simple. The only aircraft I know of that are currently in service with the concept of booster engines you put forth are certain models of the C-119 and C-123, which, respectively, have 1 and 2 100LL powered jet engines to assist in short field takeoffs, as well as the AN-24, some of which utilize around 2000 lbs of thrust generated by the APU to assist with takeoffs... why? Because they couldn't meet performance requirements with the engines originally installed in the first place, not because it saved fuel or was easier to operate.
Also, you mention "you are not carrying any extra weight around", you're forgetting about the added weight of systems for running those two additional engines, not to mention additional maintenance, checks, inspections etc.
As far as the V2500 and CF34 (I assume that's what you meant) why would I (or anyone else) want to add the complexities of what you're suggesting when you can re-engine the aircraft with a later V2500 (like the V2533-A5) and have all the gains in power, reduction in fuel consumption and none of the headaches?
Finally, the reason this concept isn't going anywhere isn't because of what I or anyone else do or don't think, it's simply because all the aircraft on the drawing board to those nearing certifications, which are expected to be manufactures for the next 20-40 years are not employing this concept.