Probably the cheapest way is to buy two 5 watt stereo amps. You want the amps running in the lower part of their range for a clean output especially on low cost equipment so 1 watt amps are too small for the job.
Connect one speaker to each channel of the amplifiers so that's four of them used.
Then connect one speaker as a third channel speaker between the two stereo speakers of each amplifier..
To do that take a lead from the positive terminal of one channel to the pos of the third speaker and then from the neg of the third speaker to the pos of the other channel.
You can use the speaker terminals for that or the speaker output posts on the amp.
That arrangement is called a Hafler circuit. The third speaker plays the difference signal between the left and right channels so you get 3-channel sound.
Of course with a mono input it won't work because there's no difference between the two channels.
For a stereo input to each amplifier all six speakers are then in use and you can arrange them around the room.
It works on all common-ground amps but not on bridged amps that are common in cheap high street music centres.
In the film industry that third speaker is called the anchor and placed in the middle behind the screen because it stops the phantom centre of the stereo image swaying too much and so film audiences get a more relaxing and balanced sound while still allowing sufficient stereo signal so a voice from the right is still from the right.
On widely spaced speakers in the living room it fills the 'hole in the middle' that wide spacing can cause and as a rear speaker it can have some weird and wonderful effects in some rooms especially if it is placed high or angled upwards and you get vertical and horizontal soundstaging with sweet spots in the room having 3D sound that floats around from nowhere, just exists.
Walk around the room and listen. You might get a trumpet just under the lampshade or a violin popping out of a gap between two cupboards with the rest of the sound distributed everywhere but coming from nowhere..
No two rooms are the same so you only now when you've got it set up just what the sound will be like and different positions for that third speaker make a big difference too..
Make sure you go pos to pos on those third speakers. Accidentally going pos to neg will do no good at all.
More on here about that and Hafler circuits. http://uk.answers.yahoo.com/question/index;_ylt=AgqsSNs5d6ZuDxVhJEe0AnwhBgx.;_ylv=3?qid=20120626035028AAbPfeG
EDIT@lare...You can get 1W speakers from RS Components and loads of other suppliers, and miniature 1W amplifiers. They are often used as embedded sound systems in lab and other instruments as well as for normal audio.