I'm another one who mainly likes to play a 3 string set-up - for all the reasons that others have mentioned, particularly the evenness of tone across the strings, and the clarity, expressiveness and responsiveness of a single treble string (rather than a pair) to a variety of techniques such as hammering on, pulling off, vibrato etc. I'm also not convinced that 2 strings are significantly louder than one. One string is well capable of standing out against open drones in older styles, including noter style playing. I'm not an engineer and I've never conducted any measurements, but I would tentatively suggest that adding a second string boosts melody volume by not more than about 10%. It would be nice to hear from the scientists among us about this!