When an amp quoted as higher impedance (15 Ohms) is used with low impedance speakers (3-4 Ohms) the amplifier will tend to overheat. The amplifier will try to dump too much power into the speakers. More than it is designed to do.
A true 15 Ohms amp is unusual these days. It's quite an "old fashioned" rating. Impedances have gradually come down over the decades from 15 to 8 and now 4 Ohms is quite normal.
Speakers also vary in impedance depending on enclosure design and frequency. Something rated as 8 Ohms might dip to 4 Ohms somewhere in its response. It follows that a 3 Ohm speaker might dip to 2 Ohms or even less. Such speakers are described as "hard to drive". Though there are other factors involved.
Ampifiers vary enormously on their ability to drive low impedances. Some can & some can't. The manufacturers won't usually tell you which. Unless they enjoy driving low impedances. Like the giant American amps.
8-15 Ohms rating means you can actually use 8 Ohms speakers. But at a rough guess 4 Ohms would be pushing it. It also depends if you like high volumes. If you run it at a low level it won't do any harm as so little power involved. But if you want to use it for typical AV levels you might very well fry the amp.
So basically it's a question without a clear answer. It depends...
A question for you: Which speakers are you considering that are rated as 3 Ohms?
cheers for the info, the system is actually for the mother and little brpther to use in the sitting room, hence the cheapy cheap approach. I have a Spny STR-DE495 going spare and have been offered a brand new set of florr standing aluminim speakers from the Sony DAV-S880 for arounf £60. The speakers are 3ohm and the amp is rated 8-16,
i understand that and im getting an active sub as the amp wont support the passive one, i was more worried bout the damage that would be dne to either the amp or speakers because of the impedance difference.