I think you need to put this into context - you've had sex 3 times. This IS something you can learn to control but it will take time and experience. At this stage I really wouldn't worry too much about it. Concentrate on getting your girlfriend to orgasm before it even goes in and then she's satisfied, lubricated and the pressure is off.
Imagine how daft you'd sound saying you'd had 3 driving lessons and you were worried you couldn't drift a car around a corner and do handbrake turns! You've got a good 50 years to get sex right - you're in the VERY steepest part of the learning curve.
Enjoy!