When to use Has, Had or Have?

First, you have to make sure that you understand the tense that you are using. For example; If you use has alone it is referring to the "third person" and saying that that person possesses something.
(She has a beautiful car.)
When we use have we are either talking about ourselves or the general idea of possessing something
(I have a great husband. It would be nice to have a bigger house).
Had on the other hand is referring to something that was possessed in the past...but not now.
(I had a good job...but now I don't).
Generally speaking the only difference is the time frame or tense that you need to use and also who is being referred to.

Here are some more examples to help you see the difference.

Have

I have a great job.
I have an Oxford dictionary.
I have blue eyes.
I have a great boss.

Has

She has a horrible job.
He has the biggest oxford dictionary.
She has green eyes.
He has a mean boss.

Had

I had a great job before the economic crisis. Now I am unemployed.
He had an Oxford dictionary but someone stole it last week.
I had blue eyes when I was born but they changed to brown when I was two years old.
I had a great boss in my old company.