hi i have a object that looks like this 
however when i add mesh collider to the 3 torus they are not working and i have convex checked as well. Only the middle portion(the joint at the very middle) is colliding. this object is composed of a middle joint, 3 oval cylinder, and 3 torus, the torus cant not collide. please help
By 'can not collide' I assume you mean, the hole in the torus must not collide, but the torus itself must? Did you try unchecking convex? Also make sure the normals on the torus collider mesh point out. Worst case, you could approximate the torus with a handful of cylinders (the collider mesh, not the render mesh)
If you do not need the exact geometry and the holes in the toruses for collision detection you could make a Compound Collider of three Capsule Colliders.