Can a 4 ohm receiver drive 3 ohm speakers?

If you connect four four-ohm speakers in parallel, the impedance will be equal to 1 ohm, and will allow the speakers to run at the advertised watt RMS. The equation is 1/(total impedance) = 1/(impedance 1) + 1/(impedance 2) and so on.

Parallel is like this:

Power source: +

Speaker+: + + + +

Speaker-: - - - -

Power source: -

However, usually if you hook up a 4 ohm speaker to a 1 ohm amplifier, it would still function it would simply not provide nearly as much wattage as it would have otherwise.

Also, if your speakers are dual voice coil (like many subwoofers) you could use 2 4 ohm speakers, connecting the two voice coils on each speaker in parallel and then connecting the speakers themselves in parallel.