Has/Have/Had is used in perfect tenses to denote the time when the action takes place.

For example:

Riya has finished her homework. This means that she finished her homework recently 

Has is used with he/she/it

Have is used with I/You/Plural. For example, I have finished my homework

Had is used in Past Tense. I had/She had/He had finished my/his/her homework when the guests came home. 

(It means that the action took place further in the past.)

It can also be used to show that something belongs or belonged to us. 

For example: I have a blue pen. Here, "have" means that I own a blue pen