John, the cheapest wireless router I've seen is £12 including delivery.... it's part of the Fon idea where they sell you a subsidised wireless router as long as you promise to share some of your bandwidth to allow other Fon users in your vicinity to connect.
There's a thread about Fon somwhere on the forum, otherwise check out
www.fon.com
As well, you'll need wireless network cards in the 3 PCs that you want to be able to connect to the router. You can buy either PCI versions, provided each PC has a spare PCI slot, or alternatively you can get USB variants - these can be good if not all 3 PCs will be connected simulataneously as you can share 1 or 2 USB adapaters amongst the 3 PCs.
Expect to pay anywhere between £10 and £50 for each USB adapter, or £20 to £60 for a PCI adapter. Personally, I prefer the PCI versions, but that's just my opinion.
However, buying the cheapest may not be the most reliable, and it could turn out to be a false economy as you might end up replacing it with a more reliable and expensive brand anyway.
Hope that helps.