try just using a diode or two in series with vdd ... each diode should drop between 0.5 and 0.7 volts
problem with using a regulator, is once your batteries aren't super fresh, you'll drop below the headroom voltage, and your regulator will either stop working all together or start oscillating or other weird anomolies
alternately you could switch to 4 rechargables and have 4.8 volts
or if you want a good hard challenge, look at the TPS61040 from Texas Inst... it's a boost regulator, takes 1.7 to 6 volts in and can be programmed to give a steady 3.3, 5, 12, up to 28 volts out.