I make this sort of thing for a living. Presumably your sensor will spend most of its time asleep, only waking up to take a reading or send data periodically. In that case a linear regulator might be a better option.
For example, a TPS78225 would give you 2.5V @ 150mA. Keep in mind spikes in current demand can be met by caps and inductors. This device has a 0.5uA quiescent current, so if you device spends most of its life asleep you should be able to get down to the single digit uA range.
Rather than using 3 cells consider using either 4 (2x2) or perhaps a different chemistry (lithium will give you 3.6V per cell). Obviously you need to minimise the voltage drop for the LDO.
This method will likely be much more efficient than any switching system if your idle current is very low. You could even do a hybrid solution where you use an LDO for the microcontroller and switching regulator that is powered up only when needed.