Hi Anjung,
Your present configuration of 4/4/4 suggests that you are already suing LAPD concentration with a factor of four.You will need to get another E1 to carry out the upgrade which you are thinking of.
Using Cabinet1 for sector A and cabinet2 for sector B and C while cabinet3 is for the rest of sector C is quite workable, but are you sure you will not need to upgrade the other sectors to 6 TRXs later? If you will, then consider using one cabinet for each sector.