Could my grinder cut a 3mm steel plate
Do you think my grinder will be enough for this? Should I buy new specific discs?


3mm? A number of abrasive wheels can cut that easily.


  Do you think my circle will have many imperfections? how could I refine it?


Depends on the diameter of the circle and how much control you have. Angle grinders have a high rotational inertia and wouldn't be my first pick for trying to cut a perfect circle. Maybe a smaller tool like a Dremel, or maybe a cutting torch might be better. I would practice on a piece first if you can.