Alex. 3-ohm speakers are not common and in my opinion, a little low but probably used since it allows the manufacturer to rate the amp at a higher power output. In my experience, while low impedance drivers do permit the amps to push more power, they do so at the expense of increasing distortion figures too to out of this world numbers. Increasing the impedance will likely result in lower distortion with little or no degradation of "usable" or practical power.
The most common specs you will see are 4 and 8 ohm impedance drivers, and 4-6 ohms is what I would recommend. You can even go with 8-ohm impedance drivers and if you can find some ultra high efficiency 8-ohm variety drivers, you might notice lower distortion and more power too. Lucky for you, 3-piece boxes typically are very flexible and forgiving when it comes to driver selection since the cabinets don't have space issues and constraints like 1-piece boxes.
Generally, the rule is to use the same or higher impedance drivers. Do not go lower than stock unless the owners manual states you can use lower impedance drivers, which would not be likely since manufacturers usually use the lowest impedance drivers they believe their amps can safely handle.
Thanks so much Super!!! Do you think 10W would be good enough?