Unlimited. When we moved house recently our main internet connection was my phone, and I didn't hold back on streaming HD video at any point. Racked up close to 30GB in that month, and had no problems at all from Three.

AFAIK I have previously used several hundred GB in a month over Three (when moving house) and it wasn't throttled at any point. Anecdotal, but I'm not aware of an obvious limit.