Summation Approval for multiple DFFs and total

Summary: Business user wants to set the approval based on Multiple DFFs to be on top of the header total so for example The 1st DFF= 20$, 2nd DFF= 30$ 3rd DFF = 20$ and the PO total with VAT is = 30$ so the Total is 100$ and they want to have the Approval to be on the Summation of the DFFs and the total (100$) instead of the seeded Total (30$), I have tried to create an expression from BPM as follows but it's not working

"new BigDecimal(PurchasingDocumentHeader.attributeNumber1)+(PurchasingDocumentHeader.attributeNumber2)+(PurchasingDocumentHeader.attributeNumber3)" Same or more than "new BigDecimal(5000) "

Appreciate your assistance

*UPDATE

I Tried it on PO Total (30$) and One DFF(20$) and it worked fine, However whenever i add all the DFFs it seem to be stuck to the 2 conditions i defined earlier (30$+20$)