Passively Buffer Voltage
A simple high-side MOSFET switch (P channel) which is switched by multiple (wired OR) N-channel MOSFETs, should do the trick:







simulate this circuit &ndash; Schematic created using CircuitLab

The P-FET is normally off (R1 sees to that). When the switch is in a position to power either of the N-FETs (only 2 shown here - OFF position not shown - just add more in the same way for other positions on your switch (CircuitLab only has DPST)) then it pulls the gate of the P-FET low turning it on.

That gives you full isolation of all the switch positions from each other, low power consumption (near zero when off, and tiny amounts when on - the biggest drain is R1 - you can increase that up to 1MΩ if you like, though it will slow down the switch-off of the P-FET somewhat).

C1 is just a big reservoir to stop the MCU blacking out when you switch the switch to a new position (break-before-make = interruption in power).

When choosing your MOSFETs make sure the \$R_{DSON}\$ of the P FET is nice and low (the N FETs don't matter much) and be sure the threshold voltage (\$V_{GS}\$) of all of them is no more than around half your supply voltage (1.5V) to ensure good saturation.