Originally Posted by
Jeff Wills
Or a double front derailleur- it'll have a cage contoured such that it'll clear all 3 rings. Or a mountain-bike front, since 42 teeth is what it's designed for.
Your still going to have interferrence with the inner cage plate if the diameter differences are too small. It's no the absolute size but the difference that matters.
Or use an old-style front derailleur with a shallower cage.
They aren't that easy to find these days. The demise of the need for half-step gearing made them obsolete.