Speakers come in 2,3,4,6,8 ohms... It can hurt your amp to drive the wrong load.. This is more true with going to a lower ohm, but can happen the other way.
An amp’s power output tends to increase as the load impedance in ohms decreases. You might see a spec of 400 watts into 8 ohms, 600 watts into 4 ohms, and 800 watts into 2 ohms. Why does that happen? As Ohm’s Law states, Current = Voltage divided by Resistance. The smaller the resistance, the higher the current. So low-impedance loads draw more current than high-impedance loads.
Here’s an example. When you play a 1 kHz tone through a power amp at a low level, it puts out a constant voltage, say, 20 volts. What is the current drawn by an 8-ohm, 4-ohm, 3-ohm, or 2-ohm loudspeaker?
Volts / Resistance = Current
• 20 volts / 8 ohms = 2.5 amps
• 20 volts / 4 ohms = 5 amps
• 20 volts / 3 ohms = 6.67 amps
• 20 volts / 2 ohms = 10 amps (assuming the amplifier can provide 10 amperes)
Power = Voltage x Current. So the three loads would receive this amount of power:
• 8-ohm load: 20 volts x 2.5 amps = 50 watts
• 4-ohm load: 20 volts x 5.0 amps = 100 watts
• 3-ohm load: 20 volts x 6.67 amps = 133.3 watts
• 2-ohm load: 20 volts x 10.0 amps = 200 watts
As you can see, the amplifier power doubles every time the load impedance is halved. That’s at low levels, where the power supply is not forced to produce much current.
However, to answer your specific question: You can use a 4 ohm speaker in place of the 3 ohm. As a flip side, you can get two lower power 6 ohm and wire them in parallel and the system will see 3 ohm. Don't go to 2 ohm.