In the normal course of things the following is the procedure.
It is entirely up to the members to decide how the assets and liabilities are to be divided among the three societies. As you are already maintaining separate accounts for the three constituents the division should have been easy. But you have the problem of FSI.
At first you should call an Extra-ordinary General Body Meeting and pass a resolution to trifurcate the Society. If the resolution is passed you can at the same meeting co-opt three provisional Managing Committees for the three resultant Societies. This can be done provided this was included as an item in the Agenda of the EGM.
The duly elected Managing Committee of the Society (prior to division) will continue and the provisional committees of the three divisions will have the status of sub-committees, subordinate to the main Managing Committee. If your existing Managing Committee does not give balance representation to the three buildings the General Body can ask for re-election.
It will be the task of the Managing Committees to divide the assets and liabilities in a fair and equitable manner acceptable to all the membership in general. If there are creditors to the Society it is necessary to decide how the liabilities to the creditors shall be shared and the creditors concerned should accept the proposal. This is an absolute must.
Among the assets there will be tangible assets and there may also be intangible assets. As you are maintaining separate accounts and probably also separate balance sheets (assets and liabilities statements) the separation will that much easy. But there may be common assets like, say, a compound not separated by boundary walls or a Society Office. The committees should decide how to divide them or compensate them among the three buildings.
Unequal FSI rights are intangible assets. The Committees will have to decide how to divide the FSI rights or provide adequate compensations. It may also require approval of the concerned authorities.
In the division in the normal course members from divisions A and B will suffer in the event of re-development. Hence obviously they will not agree for the division unless you draw up a formula to adequately compensate them. It will be entirely up to the members to decide how and in what manner the compensations would be given. You can appoint an expert consultant acceptable to all the three divisions to work out the scheme. It will be a part of the division of assets and liabilities.
After the scheme is finalized it should be passed separately by the three constituent general bodies and also by the General Body Meeting of the combined Society. Do not try to steam-roller things as you can get mired in litigation started by any of the dissatisfied members.
There are also other obvious things like selection and adoption of three separate names, three separate list of members and three separate bye-laws. These have to be done by the three separate constituent general bodies.
The three separate proposals for Societies should be submitted to the Registrar for the dissolution of the original society and registration of the three new societies.
Until the process is complete and the Registrar issues orders, the Society will continue as one Society with its own Managing Committee elected from time to time.
After the trifurcation elections to the Managing Committees of the three Societies should be held within the stipulated time under the law.