The Planet 3 services can be accessed on your 3 phone by pressing the button on the home screen, which launches the phone's default browser.
But what if you want to use Planet 3 using Opera Mini?
The short answer is, you can't. The author of this post thinks that it's due to the server restricting access to the built-in browsers, but I don't think this is the correct reason.
Opera Mini, according to Wikipedia, uses a proxy server hosted by Opera that renders the page in a special compressed format. Now, it should be axiomatic that Planet 3 is not accessible outside the 3 network, and the proxy servers are definitely located outside the network. Q.E.D.