I got a text saying I was at 75% of my data usage. I have a plan where I have up to 3g a month. If I go over, will I be notified? If I go over, will my data or my phone be shut off or will I have to pay extra fees?
You will have to pay overage if you go over your limit.
You should receive another notification when you hit 95% of your limit. But they don't/can't count data usage as real time ( there is delay of usage update), so I would be very careful after I received a notification of 75%.