I have a situation that is probably quite standard, but I don't know the right way to act.
For several months I was looking for a postdoc job in computational neuroscience (probably exact field is not that important for the question but just in case) worldwide.
A couple of months ago a quite famous professor from a distant country (Japan) after a Skype interview told me that he is interested in me but he won't take the final decision until I give a job talk at his institution A. However he was busy for some time after our interview, so his institution bought me a airtickets (quite expensive ones) for a date after about two months. I have promised to inform him if I have other job offers.
Later I have actually received two other job offers from institutions B and C (both from USA), and I like one of them. So I said "yes" to the offer from B and they started to do the paperwork although they told me it should take a week or two for them to prepare an official appointment letter.
Here comes the problem: I was told by my colleagues that I should not turn down the job talk invitation from A and another offer from C, before I get the paperwork with B finished. However it is likely to be finished after I have to go the interview to that distant institution A. I have told the distant professor that I got an offer from B and that I put it on hold (at that time I did not finish making a decision yet). Later I realized that even if A professor makes an offer for me, I the offer from B is better, but I did not tell it to A yet. Professor from C also wants me to answer as soon as possible and wants me to discuss future projects.
So if I tell "no" to A and C now, there is a possibility that the paperwork process with B hangs for some reason (for example suddenly they may receive a better application from somebody else during these weeks) and I will be in an awkward situation. If I keep waiting for the paperwork in B to be finished, I would have to lie to A and C, and I would waste A's money on my ticket (though I would be happy to come to A and give the talk in any case, on my side), which is probably a not very scientifically moral thing to do.
Maybe I am wrong in being suspicious towards institution B, but I got an impression that postdoc position market in the US is very tough and everything might potentially happen and I do consider that it is a big luck for me to have an offer from B, as it is a quite famous place and I think there should be people with stronger CV's than mine, who might be interested in this position, announced publicly.
Could you please advise me, what would be the more academically correct way for me to act?