No daily limit. Use all 3gb at once if you want. Once it runs out, you have no data connection for the remainder of the time until you recharge. You can choose to recharge before the month ends, but note that the expiry date for calls/SMS is max 30 days. Doesn't add those extra days on if you recharge early.
* I am a Boost Australia customer. This is a peer to peer community where users help each other.