how to represent 3rd library in class diagram

The notion of class and other element of static diagrams (classes, relationships and the like) has nothing to do with classification of components (3rd-party or not). It just makes no difference. On deployment diagrams, you can depict it somehow; it effects some development plans, and so on.

But to you understand that the way you draw all those lines is barely related to the development? It is not something scientifically fundamental. This is only a way to help you to organize yourself, perform analysis, validation, use as a documentation, reference for implementation, requirement document, and so on.

This is like using some common language, lingua franca, something everyone would understand, no more. It's a pretty usual mistake to consider UML and similar standards or attempts to formalize architecture, design and technical documentation as real science, a result of some mathematical speculations and proof. As a matter of fact, there is a lot of arbitrariness in all those constructs, merely reflecting the state of mind and real experience of many authors of the time when these standards/norms were emerging. It would be dangerous to canonize all this matter.

—SA