I have existing RouterOS 2.9.
Is it possible to upgrade from 2.9 to RouterOS 3.0 ?

tx guys...

Yes, it is possible.
Intall the latest 2.9.46 version and check 'system license print' for upgrade-period, if it is v3.x, then you are able to ugrade.

... after that upload the new files and reboot. see:

http://wiki.mikrotik.com/wiki/Upgrading_RouterOS

Is ver 3.rc 04 ready to be used in actuall network? Or good for trying only?

I'm using 2.9.45. Not very good at handling the router yet :">

Cheers

I try RC1 to RC4 - this version are not stable, used more CPU time than 2.9.xx.

michalkos, we need more detailed information about CPU load and test you have made.