If there was an option to record and post the audio of my scream full of joy I would do just that. These games are ridiculously great but unfortunately they never quite stepped out of the shadow of their bigger brother, Half Life. It's always that people discovered Portal through HL and never the other way around
I would love love LOVE a thitd game. But I would be also extremely worried if they announced it. It's been more than a decade since Portal 2 and I suspect many people responsible for the first two games are not even working for Valve anymore or just don't have quite enough steam (heh). Also, Portal 2 had a very definitive ending. It would be hard for me to be won over by a Portal game that doesn't have Glados in it, she and Chell/player had such a beatiful dynamic, anything less than that would be a disappointment for me. Still, in Valve I trust, and I would be very optimistic for a new game set in that universe