What are the chances of there being a portal 3

Few. After the ending of Portal 2, I doubt there would be a problem leading to Chell's Re-Entrance into Aperature. Since Glados was erased, it would make for a good storyline, but would re-hash Portal 1. Portal is still a good game, and it would always be dumb to put a money making game to rest. In the end, it's their choice.