That's partly the reason I posted it here - I couldn't find any info start with and was beginning to think it was my phone until I found someone else on Three.
I thought the same. I have unlimited data so I knew I hadn't hit any limit but thought maybe something was up with my phone. Rebooted several times. Reset network settings. etc. and nothing. I went to call them this morning but when I checked with a few colleagues also on Three and they had no data either I figured it was an outage. Annoyingly they didn't put anything on their site until after I saw this post.