Ah so that transformer thing isn't necessary? What is impedance-matching for? By three speakers I mean having the two speakers it came with (one on left, one on right) and then plugging my loudspeaker into either left or right (let's say left for something concrete) meaning I'd have one speaker on right and two on left. Is that OK? There is no "center".
impedance-matching is for - lets say the amp has a 8-ohm and a 4-ohm output (A PA amp will usually have 4-ohm, 8-ohm, and 90v), you would then match which output(s) you use to the inputs on the speaker-transformer.
You could also do stuff like 2 4-ohm speakers in series and connect it to the 8-ohm amp output.
You can use it if you wish, but don't really need it in your case.
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No don't do 2 on the left/one on the right, keep whatever number balanced on both!
If you want to use 3 speakers, you'll do what's called a "false-center"(not sure if that's the exact right term, but its technically what it is!)
Hopefully this will make sense (but I could draw it up if you needed):
Take the '+' from each stereo output, and connect it to the '+' of your 2 normal speakers.
Then take the '-' from each speaker, and connect them both to the '+' on the 3rd speaker.
Then connect the '-' from the 3rd speaker and connect it to both '-'s on the stereo.
Technically you could also create a "false-rear" by connecting both '+' from the stereo to the speaker (1 to '+' 1 to '-' ) but the speaker needs to be 16 ohms or more _and_ such a setup is not compatible with all stereos/amps.