As early as this evening, claims WSJ. http://www.bit-tech.net/news/hardware/2015/04/22/google-mobile-network-launch/1
Who's that with, Three? I pay £13-ish with Vodafone for 900 minutes, unlimited SMS and 500MB of data - but I rarely hit anywhere near that level, so I don't mind so much. In fact, I used to be on a contract with 1GB but downgraded it to save a bit of cash!