The three ohm thing sounds weird to me too. But apparently
As DPR said, replacing a 3Ω speaker with an 8Ω should not present a problem; it actually lightens the working load on the amplifier. I am not sure if this is what DPR was saying, but with everything else being equal, you will not get the same
sound pressure levels (SPLs) with an 8Ω speaker as you would with a 3Ω speaker on a 3Ω amplifier. Amplifiers double their output (in wattage) at half the Ω load: an amp rated at 100 watts @ 8 Ω will output 200 watts with a 4Ω load. Your 3Ω amp power output of 165 watts per channel would only output 82.5 watts at 6Ω: I don't remember the calculation exactly, so I can't tell you what the 8Ω output would be. But, as you mentioned about the marketing hype, a lot depends on the TRUE specifications.
And if you want to get real geeky about it, if you run two speakers in parallel off of one channel, that "sort of" cuts the collective Ω rating in half: two 8Ω speakers connected in parallel creates a 4Ω load on the amp. I say "sort of" because the correct calculation is "the reciprocal of the sum of the reciprocals of the speakers":
1/8 + 1/8 = 1/4: two 8Ω speakers in parallel gives a 4Ω load.
The reason this formula is used is for when you have different rated speakers. If you ran an 8Ω speaker and a 3Ω speaker in parallel:
1/8 + 1/3 = 11/24: a 3Ω and an 8 Ω in parallel gives an approximately 2.18Ω load.
OK: I've gone way to far for this discussion...