From the Material HIG:
Dialogs should not include more than two actions. A third action, such as “Learn more,” navigates away from the dialog, potentially leaving the task unfinished.
Avoid using a “Learn more” action to access help documentation; in-line expansion within the dialog should be used instead. If more extensive information is needed, provide it prior to entering the dialog.
Note that the word "actions" here only refers to the final actions at the bottom of the dialog. The area above is referred to as content and may contain relevant actions (like choosing a date) as long as they don't navigate away.
As for what kinds of actions you should present at the bottom, it goes into that too:
Dialogs present a focused and limited set of actions, which are generally affirmative or dismissive.
- Affirmative actions are placed on the right side and continue the process. Affirmative actions may be destructive, like “Delete” or “Remove.”
- Dismissive actions are placed directly to the left of affirmative actions and return the user to the originating screen or step in the process
Those are your "Assign" and "Cancel" buttons.
"Clear" belongs higher up, in the content area.
A dialog may have more than two actions, there is no restriction/ guideline for just two. However, you must make sure that the user doesn't accidentally press an action. Generally, what you have to think is whether it is really important to have those three actions. Is it going to confuse users? Are they really going to find it useful and make use of it? If the user is able to un- tick the name by clicking it again, maybe it is not so necessary to include the "CLEAR" action...