Author order - seniority vs contribution

Assuming you are a junior researcher like a PhD student, I would defer to the professors to sort this issue out themselves. Voicing any opinions about this matter could have a non-zero probability of stepping on someone's toes.

Given that, there is essentially no perceived difference between second and third author. And in the two possibilities you just described, all authors 'get their due', so there is no issue about fairness here either.