Heads up! I hope this is legit!

Basically: Tom Parsons tries out the HTC Vive VR headset at MWC and mentions Portal 3

Source: Stuff.tv
https://www.youtube.com/watch?v=f2HeKO-UivI&t=152

Update: Additional sources are calling the new portal demo "Aperture". Looks like a lot of work has been put into making a new portal experience for VR. While I hope this leads to Portal 3 or the press knows more than they are able to ldead on, this is only a demo that provides a new Portal experience.

Engadget quote:

"HTC and Valve saved their best demo for last. The spokesperson told me that he would keep quiet from then on and I was to follow the instructions given. As the scene faded from black, I found myself in a very familiar environment. I could feel myself smiling, grinning from ear to ear. I couldn't help myself. I was in an Aperture Science testing facility. Yes, I was inside the world of Portal."

Source: http://www.engadget.com/2015/03/04/htc-vive-hands-on/Source: The Verge

Additional source: http://www.theverge.com/2015/3/4/8146523/htc-vive-valve-vr-headset-hands-on-preview

Lies.. There is no 3 at Valve

Ooh, exciting! Maybe we will hear something tomorrow?

More like Portal 2 Ep 1

Ill believe it when I see it.

Wut?!

Could be it was a demo of portal 2, he mis-said three and meant they will announce the 'demo' tomorrow.
Valve tend to announce games more epicly than that.

Sheeeeeeeeeeit

Could be. To me he looked hesitant, like he wasn't sure if he was allowed to announce it. It could go either way, which is why I labelled the thread as a rumor.

I would love a Portal 3. I want to believe!

Update OP

Exactly.  So expect a Portal 2 Episode 1 & 2.

OP updated for accuracy.

/sad panda