Most three-speed hubs are not up to the potential axle loading or torque of tandem use. Yes, I know Electra builds three speed tandems today, and back in the old days Sturmey did offer tandem rated hubs, but AFAIK these days the only similar IGH that's blessed by the manufacturer for use on a tandem is SRAM's P5 Cargo (and only the special Cargo model, not the regular P5).

It might work fine for you, I'm just sayin'.

tcs