I've been to the edge and then come back. I've converted and deconverted most every bike I've owned to a 3 speed at one time or other. This has been my experience:

A 3 speed hub is pretty heavy. I tried it on some of my lighter bikes like my Motobecane Grand Record and Raleigh Supercourse and the bike felt sluggish. It was like I put a tractor engine into a sports car. When I put a 3 speed hub on my Schwinn Sports Tourer and Motobecane Super Mirage, it felt fine. Those bikes are both heavy bikes and performance was similar to stock.

I tried to run a derailleur on an AW with 2 cogs & a homemade JB welded 5 speed cassette on a splined driver, but I could never get enough chain wrap to keep the chain from skipping under load. Other than the novelty of it, there is really no point in adding a derailleur(s) to a 3 speed. You are defeating the purpose of a clean chainline & bike.