If these regulators use commodity ICs for their internals, it's safe to assume any extra voltage just gets dissipated as heat. Put another way, whether you are 2S or 3S, a 1200mAh pack will still give you 1200mAh at the output voltage.

Now, if they called these DC-DC converters I would have a different opinion, but those are much more complex (read heavy and expensive) animals than regulators.

Bottom line is its unlikely 2S->3S will do anything except add weight.