Am not good at abap, so you can explain your requirement to abaper.
He can develop required code to fill the target field at transformation level based on 3 incoming info objects.
Say like this we have 3 source fields, by using 3 soruce fields i need to get the result.
result = ZCONT_QTY1-ZTKT_QTY2-ZCANC_QTY3.