[BrianG]

I would say 3 NiMH cells=1s lipo. 3 NiMH nominal = 1.2v and max = ~1.4v. Put three in series, that's 3.6v nom and ~4.2v max. A 1s lipo is 3.7v nominal and 4.2v max. But as others have said, the voltage drop of NiMH is higher so you'll need an extra cell or two to get the same nominal voltage under load.