Re: BMW Tri-Power:
I think the mechanicals wouldn't be so hard (ummm, relatively speaking); it would be the electronics magic that would require some real effort. You might want to enlist the help of one of the experienced piggyback tuners. Unless you already have some good experience programming/overriding engine computers in forced induction applications).
As far as intercoolers and such...
In general, I'd say, to make for the neatest solution, you remove and/or replumb the two existing turbos into three 2-cylinder-powers-1-turbine setup.
Another possibility-->keep the stock twin snails, but add a big "top end/afterburner/ohmygodimgonnadie" third turbo, for monstrous kick at high RPM. In this case, you'd still have to replumb the two existing turbos, I think.
This setup would be some sort of Frankenstein hybrid between a 335i-style twin turbo and a 335d twin turbo (big + small).
If you're going crazy, you might as well go two or three intercoolers, depending on if you're going with the three-equal turbos (three FMICs) or the giant plus two smalls (two FMICs).
Always laugh when you can. It is cheap medicine