You need to be connected to Three's mobile network to access Planet 3. You may need to disconnect your Wi-Fi and reconnect it to the Three network to do this.
Usually, you can go to your phone's settings menu to switch off Wi-Fi.
If you're on the Three network and you're having trouble getting online, go to your internet browser's settings menu, click "clear cache" and try again.
Are you connecting over another mobile operator's network?
You won't be able to access Planet 3 from another mobile operator's network. Order a free Pay As You Go SIM to get on to the Three mobile internet network.