Tags and tenses for your examples:
Present Perfect Example
1) It's been a long time since you've seen him, hasn't it? [right, tense: present perfect] The tag verb is has.
Two Forms of the present tense with the same exact meaning:
2) They've got too much luggage, haven't they? [right, tense: simple present] the tag verb is have.
3) They have too much luggage, don't they? [right, tense: simple present] the tag verb is do
In English, there are two forms to have (meaning possessing): to have/have got, in the present tense. for have, the auxiliary is do. for have got the auxiliary is have.
So, sentences 2) and 3) have the same meaning but different forms. Have got is typically spoken not written. You can see the tags in the examples.
Word of warning: in British English, have got can be present tense or present perfect: They've got [received] a lot of letters recently. But it is not present perfect in the sample sentence; it is simple present.