I have 3 circles (2D) they all have circleCollider2D. what I want is to make a collider that make border around them. I can't express which type of collider I need, so I put example image below. Red lines are the collider I want.
Check out the PolygonCollider2D. It allows you to create a collider with any shape you want.
Another option which is less flexible but might be more performant in your particular case could be to create a CompositeCollider2D and a Rigidbody2D and then create three child game-objects, each one with a BoxCollider with "Used by Composite" enabled.