ok, it's question time...
what kind of cells are you using? I suspect you will have higher than 3V with 3 cells. why do you care about switching? for dimming?
I would think your primary concern is getting the longest life out of your batteries. any voltage over the Vf of the LEDs is wasted unless you use something like a DC-DC converter that outputs at the LEDs Vf (or slightly above). there are lots of switch mode chips made to power LEDs with a varying input voltage and very high effeciency. With one of those, you can probably drive the LEDs until the batteries are sucked dry.... You will get a lot more light that way.