No good lawyer would predict the outcome of any matter because advocacy is certainly not astrology
So the question of what are your chances of winning is not admissible of any answer
You can approach Rera
But as 3 flats are purchased, it would not be advisable to complain against delay for one flat and then think about filing subsequent Rera complaints for the other 2 flats. That is speculation and may backfire against you
A litigant has to approach the Court with all the material facts properly disclosed. Pick and choose approach is not appreciated by the Courts specially when the matter is carried in appeal to Higher Courts like High Court if you fail in the lower courts
So for the delay as regards the handover of flats, you have a case u/s 18 of Rera unless the builder comes up with a defense of force majeure to the satisfaction of the court
The interest charged by the builder is too high which too can be complained against
Non registration of agreement for the other 2 flats is also a violation of Rera
There seems to be some discrepancy in the mode of payment for the other 2 flats as informed to you while booking and while issuing the allotment letter. Did you initiate any correspondence with the builder raising this grievance ? That needs to be shown to the Court. If the Court does not find merit due to lack of correspondence for protesting against the unilateral revision in payment schedule then the Court can atleast consider reduction in the high rate of interest. 24% is quite high.