wires and 1 less speaker

Microphones, mic cables, speaker cables. Other things you might want include effects, eq, direct boxes, patch cables etc. Radio Shack (Optimus) P.A. speakers aren't going to sound that great, so some better speakers might be worthwhile at some point.

Most PA amplifiers want a load of no less than 4 ohms. That means that 2, 8 ohm speakers in parallel equates to 4 ohms. Divide 4 by another 8 ohms and you will have a loss of power, crappy sound or even a blown out power amplifier. Thankfully, companies like Yamaha already know folks who are not intelligent enough to ask the question you have have built-in protection schemes to allow loads down to as little as 2 ohms without incurring equipment damage or failure. Others aren't so lucky.

My math is lousy but at least I know how to multiply and divide in spite of my neutral density.
Mx. Remy Ann David

I sometimes use 3 speakers with my 2-channel powered mixer/PA. I put it in mono Main/Monitor mode (instead of L/R stereo mode) and drive 2 speakers in parallel from the main out for FOH, and the 3rd speaker from the monitor out as the band's monitor.

BTW - The equation for parallel impedance is 1/Z = 1/Z1 + 1/Z2 + ... + 1/Zn. eg. 3 8Ohm speakers in parallel gives 1/Z = 1/8 + 1/8 + 1/8 = 3/8 -> Z = 8/3 = 2.66666 Ohms.

Which is of course substantially less than 4 ohms when you're talking about values this small. So that's significant.

I'm personally usually loaded with something and it's not impedance
Mx. Remy Ann David