The three Brown wires are Live and should all join together and to nothing else i.e. they do not connect to your new light...
One of the Browns is live coming in (it has an associated Blue neutral) Another Brown is Live to the next fitting in the house aslo with an associated Neutral. And now it gets interesting...
The third Brown runs off to the light switch and it has an associated Blue coloured wire which is NOT neutral - it's Switched Live and needs to join somehow with the Live connector on the new fluorescent tube - so you can turn it on and off.
The first thing you must do is identify Switched Live else you'll be tempted to join it with Neutral wires and constantly blow fuses - Get a mains tester screwdriver to do this.
Then you need a 4 way terminal box in the ceiling void.
All the earths in one terminal. All the Live wires in another. Switched live on its own. and two Blue neutrals together. Then run a short length of cable from the Terminal Box to the new fitting - Earth to E / Switched Live to L / Neutral to N. Robert est votre oncle!
Because of the special nature of Switched Live it should carry a Brown Sleeve to identify it - look inside the switch housing and observe the sleeve.
Here's a drawing of a ceiling rose that gives the general idea - for clarity it does not show the feed-out cable but you can easily see where it would connect...
http://www.secelecuk.com/resources/ceiling+rose.jpg
Good luck...take care...
Peccavi
June 2009