For £50 you are really going to get nothing. 3 Logitech desktop microphones, which will have mini-jacks on the end, so maybe a combining adapter to bring that into one computer? Or a bunch of adaptors to make them 1/4", then into a mixer that isn't really a mixer (ie. not made by Mackie, Yamaha, tapco or whatever), something a bit like this.
For £50 you're not going to sound that great.
http://www.podcastrigs.com/ is a good site. But you're looking below the entry level, but it might have some ideas.