I'm seeing a lot of people misconstruing the announcement being made online, so to clarify:
Mr. McGee announced on his blog that he is working on a BUSINESS PROPOSAL for the third game. Basically, this means that he's creating a presentation to show to EA in order to pitch the idea for a third game. It DOESN'T mean that a third game is confirmed. EA has the rights to the "Alice" franchise, which means that in order a third game to be produced, they have to ACCEPT Mr. McGee's proposal first. Right now, all this really means is that a third "Alice" game is a POSSIBILITY.
If fans really want the third game, head over to Mr. McGee's blog where he made the announcement (http://www.americanmcgee.com/alice-3/), signed up for the mailing list, and if possible, give financial support to the pre-production of the project by sending money to Mr. McGee via PayPal. Doing this will show EA that there is a large fanbase that wants a third game made for the franchise. Naturally, as Mr. McGee continues to stress over the years, emailing, tweeting, and sending messages to EA in general about a third game for "Alice" is a good and quick way to show support for the project.
Yep. Spread the word through Tumblr, Facebook, Twitter, and other SNS. Tell your friends, family, and followers about the news. Fandom support is necessary for Mr. McGee to show EA that people want a third "Alice" game. Making fans aware and informing how to support it is key.
Again, though, a lot of people are misconstruing the announcement online, like on Tumblr. I'll have to go around anonymously on the tags and set things straight to avoid any confusion and misinformation, since I really don't think anyone else will.