How to connect DAC - MCP4725 to the speaker?
to achieve 3 watts (RMS is how Vrms * Vrms / speaker_impedance. we find

Power * Speaker_impedance = Vrms * Vrms = 12

Vrms = sqrt(12) or 3.5 volts rms or about 5 volts peakpeak.

Thus you need +- 5 volts at a minimum, or 5 volts with a H-bridge power output amplifier.