Combine Nexus 3 Hub with Torpedo 3 gear shifter?
According to Sheldon Brown's exhaustive list, the Shimano is fairly close to the two hubs that use the Torpedo shifter. Shimano wants 14.2mm of cable pull between gears 1 and 2 where the two Torpedo-compatible hubs want 14.0mm and 13.8mm, then between 2 and 3 it's 7.8mm vs 8.7mm and 7.7mm, which suggests that the tolerances on the hubs are sufficient that you should be able to find and adjustment that works reliably.

HUB                  -&gt; mm of cable pull                      
Shimano 3-speed Nexus IG    14.2   7.8
SRAM T3                     14.0   8.7
SRAM i-motion 3             13.8   7.7
Sturmey-Archer 3-sp trigger 14.2   9.5


The Sturmey-Archer trigger might work, but you might find it over-tightens the cable in top gear.

You could also use a handlebar-mounted friction shifting lever like this:
 

This question about the differences between the various Sachs/SRAM hubs might also be useful to read.