Managing multiple databases

Here is the scenario
I have 3 oracle databases on a single linux server. All installed under the same owner oracle. Each one supports a different language. I want to start all the three databases up and running at the same time. How do i do it? What parameters do i need to change? Do i need to set any environment variables? Or do i need to install it under different oracle user group? If so, then how? Any help/insight into this will be helpful.