Switch between 3V battery and 5V USB as power source
You might be looking for something like this, an ideal diode block: -



It's a little high power for your application but maybe there are lower power types. I'd try reading the data sheet for the LTC4415, getting some keywords then doing some googling. You'll probably find other offerings from Linear Technology and quite possible TI.

If you can manage to use a schottky on your USB 5 volt rail you could use this (a little cheaper): -