Could you please explain the rule of mutanajjis? If my left hand has urine on it and I touch a door handle, is the door handle the first mutanajjis or second? Then if my right hand touches the door handle, is my right hand the third mutanajjis or first?
What comes into direct contact with a najasah is called a mutanajjis, and it is the first mutannajis. Therefore, if the urine itself is transferred to the door handle from your hand the door handle is still a first mutanajjis.
If your left hand is wet with water and the wetness of that hand mixes with the urine and then you touch the door handle [with that wetness], the door handle is deemed as second the mutanajjis. If your right hand then touches the door handle and there is enough moisture on your [right] hand or the handle then your right hand becomes the third mutanajjis.
NOTE: if the najasah itself transfers to an object, that object is considered the first mutanajjis, which means there can be multiple first mutanajjis); if the first mutanajjis comes into contact with a pure object, that object is considered the second mutanajjis. Similarly, the second mutanajjis will make a pure object impure if it comes into contact with it, this is the third mutanajjis. However, the third mutanajjis can no longer make other items impure, irrespective of whether it is wet or dry.