What is the past tense, third person, plural form of "to have"?
Third person plural is 'they', and past tense could be one of two things, generally.
Imperfect past is 'they had'.
Perfect past is 'they have had'.
Perfect past uses an auxiliary or helping verb ('have') and a past participle (in this case 'had'). Imperfect tense has no auxiliary verb and just the imperfect conjugated form of the verb. There is also the pluperfect past, which is 'they had had', using the imperfect conjugation for the auxiliary verb.
A conjugation is just a form of a particular verb, such as 'have', 'has' or 'had'.