Not quite sure why they guys above spent more time bantering than answering your question, given that this is a DIY forum, and DIY installation/replacement of a light fitting isn't an unreasonable task.
RR came closest, but he's not named that for no reason, and a picture might be easier to follow:
It's not quite as helpful as it could be, as it shows two reds going to the switch, when in 99.999% of cases it's a length of T&E, with the red used to take the live feed to the switch, and the black used to bring the switched live back. This is supposed to be marked with a bit of red tape or red sleeving, but often this isn't done. It also doesn't show the earth wire going to the switch, which is naughty.
Anyway, you should be able to see why you have 3 cables. Rather than post more pictures here, I'll point you at this web page:
http://www.diydick.com/index.htm?http%3A//www.diydick.com/Electrics/electrical-c...where they do a good job of building up from first principles to show how all the various conductors in the ceiling rose work. It's very simple once you get the hang of it, and once you have, then if you need to replace the rose with a light fitting that has only L/N/E connections, you'll be able to work out which wires you need to join yourself with junction boxes or choc-block.
Apart from turning off the power, the most important thing you need to do, before you disconnect anything, is to look for the black wire that is not connected to either of the other two black wires, but instead is in the same terminal block as one of the light pendant wires. This is in the cable that goes to the switch, and you need to make sure you mark that so that you know which is which when you connect it all back up.