Hello,
What don't you forecast on HALB level? use strategy 70, which allows you to plan at component level.
Strategies for Planning Components - Demand Management (PP-MP-DEM) - SAP Library
Or another way to map this scenario is create 3 production version for your alternative BOM
BOM alt 1-component A (HALB)
BOM alt 2-component B(HALB)
BOM alt 3-component C(HALB)
Then create the quota arrangement for finished product and define the percentage against these production versions. So after the MRP run you will have the planned orders for all the HALB as per the defined quota.
Refer this useful document on quota arrangement.
Functionality of Quota Arrangement in Production
Best Regards,
R.Brahmankar