There's probably two complications with that plan: first, you end up with a tunnel that is too steep (you have to dip deep enough to get under the water), or yo

There's probably two complications with that plan: first, you end up with a tunnel that is too steep (you have to dip deep enough to get under the water), or you end up digging (and outfitting) four tunnel entrances, rather than three. [n.b. I am assuming the plan actually makes sense, and that when you compare the three-headed tunnel, the two tunnels, and the tunnel & bridge solutions, and all of the maintenance and externalities of roads connecting them, the three-headed tunnel is no more expensive, and trying to think of reasons this may be. If the plan is actually more expensive, or if no counter-proposals were considered, this is all somewhat moot.]