Sure, I don't think the roundabout is that special. But if building a short third leg is cheap, building an even shorter third leg (instead of one that meets the others "in the middle") should be even cheaper.
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.]
