What power supply you have ? What is primar voltage ?
Always is better to use transformer as isolation device, but if you have less space for device, you can use capacitor/resistor to lower mains voltage (if you mean on that).
Best power saving is with switch in Off state. :smile:
Transformer is best solution if you have transformer and needed space for that. Also switching psu can be very good solution.
You can decrease voltage from 220V to some LED voltage with capacitor and resistor no problem, what LED you plan to use and what current? You say 3 white led at 3,6V each ok, what current ? I supose 3 led are in serial connection.