Notice the comma after B. This is a non restrictive clause. The adjective should describe the situation (A has found B). I wouldn't describe a situation 'lucky'. I'd use the word 'fortunate'. If your B is a person, you may use 'lucky'. But I would lose 'I feel
Sorry. I just came back home from a three-day vacation.
All three are correct depending on the states of affairs. You must remember, though, that the past perfect (had + Vpp) is used with past actions or events that happened before a past time (expression) or another past action. The tense of relative clauses (here) is not really influenced by the tense of the main clause so you