Welcome to the forum!
Yeah, I think that guy probably either didn't buy it new or was messing with you - that receiver came as part of a 5.1 package (the Sony HT-DDW790). Maybe it was part of a 3.1 package, but like you - I doubt it.

If you don't have it already, you can download the owner's manual for that system in PDF format at the link
here.
Anyway, you've got all the parts that you need, now. The thing with lower impedance speakers (3-ohm versus 6-ohm) is that they'll draw more current from the receiver. The higher the volume level, the more current a speaker will draw. If the receiver has to provide more current than it was designed to do, then it could overheat and/or fail. You've got two things working in your favor, though:
- Your Sony has a protection circuit to prevent damage in this situation. If the protection circuit turns on, then you know that you should either use different speakers or not turn them up so loud
- I'm assuming that you're going to use the two new speakers for surround speakers. Those tend to have relatively little power (and, therefore, current) going to them as compared to the front speakers.
My guess is that you won't run into problems with it unless you like to play stuff at really high volumes.