Everyone has to start somewhere.
You say you have been programming for three years. Is that with a job or just doing things for yourself? Are you a computer science graduate or self-taught?
Technology is moving so fast today that I would not worry about not knowing everything; however it is important that you have a strong base of knowledge about programming principals. I actually prefer to hire engineers that I know can learn fast instead of just being expert in a single technology.
Most software engineers learn on the job anyway. They obviously vetted you in order to give you a job.
My advice is to work as hard as you can outside of the job as well as on the job to learn as much as possible. There are a lot of resources online that can help you learn things like Maven and Git. Spend as much time between now and when you start learning (and actually doing) work with Maven and setup an account on Github.
Good luck!
Source: Quora