You are measuring DC resistance with a meter which is very different than the AC impedance used in the specs.
Its normal for the DCR to run lower than the nominal or averaged impedance which varies with frequency.
3.3 ohms DCR does seem a tad low. Some speakers cheat a little to draw more power and sound comparatively louder in the display board. When you go to bridge your amplifier or run two speakers in parallel, an inordinately low impedance can cause amplifier thermal and stability issues. Additionally, a Sony amplifier is decent but it certainly won't compare to similarly rated amplifiers from the more respected domestic brands. So run it conservatively. From your description however, it sounds like you've got another issue as to why the speakers wouldn't originally play in stereo.
Do not run a combination of bridged and parallel if you plan on keeping that amplifier for a while.