Congratulations – that's exciting!
First – and most importantly – make sure you really know your idea inside out. What length is it? What's the story? Where does the story begin and how does it get to the end? Over what period are you shooting? Whose point of view are you focusing on? Who are the key characters? How are you telling the story on screen? Using talking head interviews? Ob doc? Rare archive? Animation? And why does this story need to be told now? And indeed, why does this story best told as a documentary rather than a book, magazine feature or photo essay?
Once you've got it straight in your mind you can plan your pitch. Make sure you pitch the story and not the issue or research. The execs will be trying to 'see' your documentary as you pitch it, so make it unfold in their minds with some vivid visual details to bring it to life.
And you only have 3 mins so you don't have to tell the whole story from beginning to end (they'll start glazing over)- summarize it in one sentence e.g. "This is the story of...who...and then discovers that...until... happens". You can then fill in some of the details – how you came to the story, who the characters are and what their challenges are, for example.
The pitch should be a tease that leads to a dialogue. Once they start asking questions – shut up and listen. You can talk yourself out of a pitch by not listening and responding to feedback.
There are a number of articles on pitching here: http://www.tvmole.com/category/tvdevelopmenttips/pitching/page/2/ – scroll down the page and start with the one titled "Six Ways..."
Good luck – and enjoy the experience!