These verbs belong to the category of auxilliaries which are often used to buttress or help other lexical(visible and practicable action words)verbs.
In the first instance,the difference between "has" and "have" is number...while "has" goes with singular nouns or pronouns,"have" goes with plural verbs...
NB.The pronoun "I",though it appearsto be referring to a single person or entity,it takes a plural verb..."I have"...not "I has"
The verb "had" however,reflects on tense...
I had a car when I was a student(past tense)...(1)
She had eaten the food before we returned(past perfect tense)...(2)
The verb "had" in example (1) above is not an auxilliary verb...It is a lexical one which can be substituted for a verb like "owned"...I owned a car...However,the same verb in example (2) above is an auxilliary (helping) verb buttressing the verb "eat" to show its aspect and state.