Where can I find the latest patches for raq3 and which ones are needed for security of server.

http://www.cobalt.com/support/download/index.html

or if you have an "english speaking" RaQ :)

http://www.cobalt.com/support/download/raq3.eng.html

You can see what every patch does looking at the descriptions.

If I was you I would install everything, starting from the oldest patches (at the bottom of the page you will get).

On our RaQ 4i we installed all the patches, up to the ones released in April 5. If you want you can wait a little before installing them, since sometimes released patches are not... perfect and cause some trouble.

Best regards.