I was able to connect 2 laptops using Ad-Hoc, but how to connect the third one??
The same thing as 2 laptops, on the third laptop you look for the SSID which the 2 first are connected to and that's it.
I remember myself playing a game at university with more than 15 friends connecting to the same Ad-Hock wireless ;)
There should be no difference in connecting the third as from the second.
When you first create the network, it should be broadcasting the SSID and if you connect, it should ask for a passkey.
If it isn't working, try again. Sometimes I have found that some laptops go funny and it is easier to start the ad-hoc for a different one - in theory it shouldn't make a difference, but sometimes it does!