You've asked about the difference between the first two of the following; when one would you use the first form; and how the first two differ from the last.
• I am going to have to see you sometime.
• I have to see you sometime.
• I will have to see you sometime.
The general meaning of all three is the same: I must see you, at some future time. To my thinking, there is no indication built into any of these as to how soon I must see you. Depending on context and tone, in any of these three, sometime might be made to mean right away, or at an indefinite time, or any time, or never. Likewise, tone can make each verb show reluctance, resignation, eagerness. Also, in any of the three, meaning depends on where emphasis is placed. Half a dozen nuances arise for each, as emphasis is placed on the subject, the verb, the auxiliary, the infinitive, the object, the adverb, or on combinations of same.