I have submitted my review paper for publication to more than two journals. I was unaware of the consequences. The paper got accepted at al three journals. They asked for payment and copyright etc. I haven't processed anything as I only want to publish it in one journal only. I also became aware that it's unethical. So far, I only have the acceptance emails, but no further processing has been done by me. What can I do now to publish my paper? I think that one of the three journals is best one and I want to publish my paper there. What do I have to do so that I'm not being unethical nor breaking any rules?
To get to a stage wherr your paper has been accepted, multiple people for each journal have spent their time and effort to assess your paper (editors, reviewers) - while in the case of the reviewers not even getting payed for their efforts. By submitting the paper to three journals at the same time, you have basically wasted the time of al people involved in the submission process of two of the journals. The proper way to do it (for the future) is to submit to your first choice, and then if you don't get accepted sent it to the next one in line. This is a more timeconsuming process but the morally and ethically correct one (see also the answers to this question).
As this train has already left the station (and you already behaved unethically by wasting peoples time and ressources) you should try to do damage control and inform the other two journals as polite as possible that you have to decline the offer. Don't try to make up lies (like you don't have funding anymore) as the editor of one journal might be well aware of what articles are published in the other, related ones and will probably see that your article will be published there.
You could try to be brutally honest and tell the other two journals what you did and that you are very sorry, but being an unexperienced academic you did not know better and will not do so again in the future.