If a $750,000 loan is taken on Property C, then Property C can't take any deduction.  That loan did not do anything for Property C.

However, Property B can take the interest on the portion for the $500,000.  It is essentially just refinancing the loan.

Property A can't take any deduction.  Although it is refinancing Acquisition Indebtedness, another requirement for the personal interest deduction on Schedule A is that the loan must be against that personal residence.