Well, ultimately the question is "Can I afford to lose 1 Bit of resolution in exchange for speed"?

If the answer is YES, then your worstcase scenario with a 10-Bit ADC is...

5v Sensor = 1023 Bits FSD
3.3v Sensor = 675 Bits FSD
3.0v Sensor = 613 Bits FSD.

... and you don't have to do anything.

If you can't live with that, then it looks like OP-AMPs could be your choice. That's a lot of extra components for one extra measly bit of accuracy...

But you could of course run your PIC at 3.0v (the voltage of your lowest Sensor) and simply use divider Resistors for your 3.3 and 5.0v Sensor inputs.