Multiple (>2) voltage sources
If voltage drop is not an issue you can just tie all the power inputs together with diodes:
all the cathodes are connected together, and that's where you power your circuit from, and
you hook one power supply per each anode. Issues? Voltage drop, power dissipation. That can't work for big boards where much current is needed. You can use Schottky diodes to achieve a lower drop but that pretty much is it.

If there's a some sort of precedence between the supplies you can sort of solve the problem still using only diodes. Here's a sketch:







simulate this circuit &ndash; Schematic created using CircuitLab

If PS1,2,3 are rated the same voltage then PS3 is the highest priority one, i.e. when it's on that's what's powering your circuit. If PS2 kicks in D2 is off since the voltage drop across it is 0V, but if PS3 fades away then your circuit is powered through D2. Note that when you are using PS1 you have two diodes drop to take in account.