Ann,
Very good scenario. Assume this is in Process, Chemical or Pharma industry. So assume you use the PPPI module. If Yes, then SAP offers in Recipe an option called Material quantity calculation.
Let me explain - if you want to use Material X completely for producing Material A or B or C then it allows the process order qty of either A, or B or C to be automatically be increased based on the planned consumption of X.
In your example you have 1000L of X produced. However you require only 600 L to make product A,B and C will be as per your example.
However in Process Order of C, you can increase planned consumption of X from 100L to 500L (add 400 L that is remaining stock on hand of X) and then system will bump up Process order qty of C based on BOM ratio.
This method allows you to consume complete batch qty of X in producing your 3 products A, B, and C