There is no problem with connecting a 3-ohm speaker to that amplifier. Only be aware that an impedance is a resistance expressed as a complex number, which means it has a real part (that corresponds to the DC resistance) and an imaginary part that behaves differently based on the frequency of the current going through it.
Since the two impedances are not matched, there will be some reflections at some specific frequencies.
It is highly probable that this won’t be heard on a signal below 20kHz.
However, if your amplifier is built to deliver it’s rated output to a 4-ohm speaker, then your speaker will draw more current. All things being equal, it might sound louder but your amplifier might heat up too much or if it is well-designed, you could burn the loudspeaker itself.