I want to design UI prototypes and wireframes, based on Use case diagrams, activity diagrams and sequence diagrams to represents the flow and process relation. Is it necessary to include the class diagram in the design?
Class diagrams are for:
a. Describing the low level design of a software component
b. Describing a data model.
Wireframes are for designing the interaction and experience of the user with the system. The use cases act as input for the wireframes.
The use cases can be detailed in text (actors, preconditions, main success scenario, postconditions, branches, exceptions) or in activity diagrams.
Sequence diagrams show how the use cases are implemented above the classes from the class diagrams and aren't really input for the UI design.
If you want to describe how a user interacts with a UI then use wireframes.
The class diagrams are for designing the implementation and the data model,
When you get to the UI low level design - designing the specific fields in a UI, you will want to take into account the UX design (wireframes) and the required data (data model - class diagrams) to make sure the UI is according to the desired experience and that it provides all the required inputs. You may even discover that some of the inputs don't match the desired UX and go back and adjust it e.g. by adding an additional window or control then return to the UI low level design and adjust according to the changes to the wireframe.
UML class diagramm isn't necessary for the UI design. Though, if it exists, it could be the source for the entities' atributes, which are exist in a domain. So, for example, Author and Book classes have attributes, which could be implemented in a UI. But there is no any data in the diagram which tell you what attributes should be on a form.
For UI designer who has UML class diagramm only as input spec for an UI, there is a danger of implementing UI, which strictly follows the data model, but brings bad UX. An example is implementing user registration form, which contains all the user profile attributes in the registration form.
So use UML class diagramm as additional source of information, but don't stick with it.