If the payment is made so that the applicant should be considered on merit or when all/many other applicants are making the payment and in order to stay in the race one, then there is no harm in making such payment. These measures are taken to save oneself from any injustice that might be committed against a person.
However, if the payment is made solely with the intent to bypass the rest of the candidates without giving them their due chance, then such a payment is immoral and usurpation of the rights of other candidates. Here the payment is made to not prevent injustice but to commit injustice against others.
There are some other issues that should be of some concern in this transaction:
i) How can one be sure that only three candidates will be making payment and get their seats and the rest of the applicants will be unaware of this fact and will not make any payment?
ii) If the payment is made by more than three candidates then what will be the criteria of selection of only three seats which are available?
iii) In case the seat is not allotted to one of the applicants who has already made the payment, then what legal course would be available to him other than losing his money?
It is possible that one may not have answers to all the questions but it should be kept in mind that any such financial transaction, if done by the applicant, it should be with an honest intent of preventing injustice against oneself, not to commit it against others. One has to make such decisions in the light of whatever information is available and if the decision was done with utmost honesty, then any mistake in making such decision will be forgiven (hopefully, as per Allah’s law).