I had an inguinal hernia surgery (open repair) before three years. I want to know is it possible for scar tissue not to be shown in the MRI scan?

Yes. Children usually heal well, and scar tissue can reduce over time. A childhood hernia repair is not a big procedure or incision, so the scar tissue may not be visible after a period of time.
Sure. When you are young your skin is growing and changing. You do not always get scars like adults. The type of scar you would get probably would not show up on MRI anyway.