Matching a 3W+3W audio amplifier with 2x6W speakers
The power rating of a speaker indicates the maximum power it can accept without damage. A speaker of an equal or higher power rating than the amplifier is a good idea, otherwise the amplifier at maximum volume is likely to damage the speakers.

The volume will depend on the sensitivity of the speakers. Although sensitivity varies between speakers of different construction, sensitivity is not directly related to a speaker's power rating. Meaning, a 3W speaker will not necessarily sound "louder" than a 6W speaker connected to the same amplifier. There's no particular reason to look for a 3W speaker unless you are trying to cut costs.