555 VCO Circuit - Speaker to VCC?
A speaker of 3 W and 8 Ohm could draw I = sqrt(P/R) = 612 mA, but 555 only delivers (source or sink) 200 mA as noted in:
http://www.ti.com/lit/ds/symlink/lm555.pdf

So you need a current amplifier/buffer between the 555 and the speaker. This additional stage can also be used to decouple the DC component.

Also, though not directly related to your question, you should consider using a filter to obtain a clearer tone at the speaker, as the 555 output is not a sinusoidal wave.

About whether to connect to the supply or ground:

The AC component is still there as the 555 can sink or source current, and this AC component is what becomes sound. What changes between connecting to supply or ground, is the DC component.