Well you could potentially place too much load on the amp.
You can look at OHMs which is resistance as a pipe. The lower the ohms, the larger the "pipe" for current to travel through.
If you think of the AMP as a pump, it will have to work harder to output the same current on the larger pipe.
It isn't inherently bad, but could potentially cause the amp to over exert itself over time.
If your amp is rated for 2-4 ohm speakers, a 3 ohm is perfect.